Method for preparing a hologram recording medium

ABSTRACT

Two original images to be recorded are prepared as data (S 10 ). A plurality of unit regions, each having an adequate area to record interference fringes of visible light, are defined and positioned on a hologram recording surface (S 20 ). A gradation pattern, with which appearance probabilities of two record attributes gradually change in space, is overlapped onto the recording surface, and to each unit region, one of either record attributes is assigned according to the appearance probabilities of the respective record attributes at each individual position (S 30 ). In each unit region, to which the first record attribute is assigned, the first original image is recorded as an interference fringe pattern, a diffraction grating pattern, or a scattering structure pattern, and in each unit region, to which the second record attribute is assigned, the second original image is recorded as an interference fringe pattern, a diffraction grating pattern, or a scattering structure pattern (S 40 ), and a record pattern is formed on a physical medium (S 50 ).

BACKGROUND OF THE INVENTION

The present invention relates to a method for preparing a hologramrecording medium and particularly relates to a method for preparing ahologram recording medium, with which a gradated motif expression isenabled by computation using a computer.

Holograms are widely used in applications for preventing counterfeitingof cash vouchers and credit cards. Normally, a region onto which ahologram is to be recorded is set up in a portion of a medium to besubject to counterfeiting prevention, and a hologram of athree-dimensional image, etc., is recorded inside this region.

Although conventionally with many commercially utilized holograms, anoriginal image is recorded onto a medium in the form of interferencefringes by an optical method, methods for preparing a hologram byforming interference fringes on a recording surface by computation usinga computer have come to be known recently. A hologram prepared by such amethod is generally referred to as a “computer generated hologram (CGH)”or simply as a “computer hologram.” A computer hologram is obtained bysimulating a so-called optical interference fringe generating process ona computer, and an entire process of generating interference fringepatterns is carried out as computation on the computer. Upon obtainingimage data of interference fringe patterns by such a computation,physical interference fringes are formed on an actual medium based onthe image data. As a specific example, a method, with which image dataof interference fringe patterns prepared by a computer are provided toan electron beam drawing equipment and physical interference fringes areformed by scanning an electron beam across a medium, has been put topractical use.

With the advance of computer graphics technologies, the handling ofvarious images on computers is becoming a general practice in theprinting industry. It will thus be convenient to be able to prepareoriginal images, to be recorded as holograms, in the form of image dataobtained using a computer. To answer such demands, arts for preparingcomputer holograms are becoming important and are anticipated to takethe place of optical hologram preparing methods in the future. Variousarts related to such computer holograms are disclosed in Japanese PatentPublications No. 11-024539A, No. 2001-109362A and No. 2003-186376A(hereinafter Patent Documents 1, 2 and 3).

Also, although “hologram” normally refers to an optical interferencefringe pattern that enables reproduction of a three-dimensional pattern,recently, a medium called a “pseudo hologram,” in which a diffractiongrating pattern is formed in place of an optical interference fringepattern, has come to be used popularly. For example, Japanese PatentPublications No. 06-337622A, No. 07-146635A and No. 07-146637A(hereinafter Patent Documents 4, 5 and 6) disclose methods for preparinga “pseudo hologram,” with which a predetermined motif is expressed byarraying diffraction grating patterns of various types as pixels, byusing a computer, and Japanese Patent Publication No. 2001-083866A(hereinafter Patent Document 7) discloses a method for recording such a“pseudo hologram” and a “normal hologram” on the same medium. Also,Japanese Patent Publications No. 2002-328639A and No. 2002-333854A(hereinafter Patent Documents 8 and 9) disclose examples of pseudoholograms that use scattering structure patterns instead of diffractiongrating patterns.

As mentioned above, although a normal “hologram” refers to anarrangement with which optical interference fringes of an object lightand a reference light are recorded on a medium, recently, media, withwhich various motifs are expressed by diffraction grating patterns orscattering structure patterns, are also coming to be referred togenerally as “holograms.” Thus, in the present application, the term“hologram” shall be used as a broad concept that includes not onlynormal holograms, formed of optical interference fringe patterns, butalso includes pseudo holograms formed of diffraction grating patterns(diffraction grating recording media) and pseudo holograms formed ofscattering structure patterns (scattering structure recording media).

In a hologram for a cash voucher or credit card, various motifs, such asa company logo mark, a character string indicating a company name, etc.,are recorded according to application. Methods for superposinglyrecording a plurality of motifs on the same hologram recording mediumhave thus been proposed. Because both normal holograms, in which opticalinterference fringe patterns are recorded, and pseudo holograms, inwhich diffraction grating patterns are recorded, have a function ofmaking use of the diffraction phenomenon of light to generatereproduction light, directed in specific directions, two motifs can berecorded superposingly in a manner such that a first motif is observedupon observation from a first direction and a second motif is observedupon observation from a second direction. For example, theabovementioned Patent Documents 2 and 3 disclose methods forsuperposingly recording information of a plurality of original imagesonto the same recording medium, and the abovementioned Patent Document 4discloses a method for superposingly recording diffraction gratingpatterns for indicating two different alphabetical characters.

When two motifs can thus be recorded superposingly, the two motifs canbe displayed switchingly according to the observation direction so that,for example, a motif, constituted of a character string indicating acompany name, is observed upon observation from a first direction, and amotif, constituted of a company logo mark, is observed upon observationfrom a second direction. However, depending on the application, such amethod of switching according to observation direction may notnecessarily be appropriate. For example, there are cases where it ispreferable for both the character string indicating the company name andthe company logo mark to be displayed next to each other at the sametime.

Such cases are conventionally accommodated by simply positioning the twomotifs adjacently. For example, the abovementioned Patent Document 7discloses an art of recording a first motif as an optical interferencefringe pattern onto a central region of a medium and positioning asecond motif as a diffraction grating pattern at a peripheral region ofthe medium. However, when a plurality of motifs are simply positionedadjacently in this manner, the motifs do not blend well with each otherand the resulting hologram lacks design quality.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to provide a method forpreparing a hologram recording medium, with which good blending can besecured among recorded motifs by a gradated motif expression.

(1) The first feature of the present invention resides in a hologramrecording medium preparing method comprising:

an original image preparing step of preparing, as data, a specificoriginal image to be recorded;

a unit region defining step of defining and positioning a plurality ofunit regions, each having an adequate area for recording interferencefringes of visible light, on a hologram recording surface;

an attribute assigning step of assigning a specific record attribute,which indicates that the specific original image is to be recorded, to aportion of the plurality of unit regions;

a record pattern preparing step of determining, for each unit regionassigned with the specific record attribute, an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on the specific original image to prepare data indicatinga predetermined record pattern to be formed on the recording surface;and

a medium forming step of forming the record pattern on a physicalmedium.

(2) The second feature of the present invention resides in a hologramrecording medium preparing method comprising:

an original image preparing step of preparing, as data, a specificoriginal image to be recorded;

a unit region defining step of defining and positioning a plurality ofunit regions, each having an adequate area for recording interferencefringes of visible light, on a hologram recording surface;

an attribute assigning step of defining a gradation pattern, whichexpresses that an appearance probability of a predetermined attributegradually changes in space, and assigning a specific record attribute,which indicates that the specific original image is to be recorded, to aportion of the unit regions that is selected according to an appearanceprobability at each position when the gradation pattern is overlappedonto the recording surface;

a record pattern preparing step of determining, for each unit regionassigned with the specific record attribute, an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on the specific original image to prepare data indicatinga predetermined record pattern to be formed on the recording surface;and

a medium forming step of forming the record pattern on a physicalmedium.

(3) The third feature of the present invention resides in a hologramrecording medium preparing method comprising:

an original image preparing step of preparing, as data, a plurality M oforiginal images to be recorded;

a unit region defining step of defining and positioning a plurality ofunit regions, each having an adequate area for recording interferencefringes of visible light, on a hologram recording surface;

an attribute assigning step of defining a gradation pattern for each ofthe M original images, respectively, which expresses that an appearanceprobability of a record attribute corresponding to an original imagegradually changes in space, and assigning any one of record attributesto each unit region according to appearance probabilities of therespective record attributes at each individual position when therespective gradation patterns are overlapped onto the recording surface;

a record pattern preparing step of determining, for each individual unitregion, an interference fringe pattern, a diffraction grating pattern,or a scattering structure pattern based on a specific original imagecorresponding to an assigned record attribute to prepare data indicatinga predetermined record pattern to be formed on the recording surface;and

a medium forming step of forming the record pattern on a physicalmedium.

(4) The fourth feature of the present invention resides in a hologramrecording medium preparing method comprising:

an original image preparing step of preparing, as data, a first originalimage and a second original image to be recorded;

a unit region defining step of defining and positioning a plurality ofunit regions, each having an adequate area for recording interferencefringes of visible light, on a hologram recording surface;

an attribute assigning step of defining a gradation pattern, whichexpresses that an appearance probability of a first record attribute andan appearance probability of a second record attribute gradually changein space, and performing a process of assigning one of either the firstrecord attribute or the second record attribute or not assigning eitherattribute on each unit region according to appearance probabilities ofthe respective record attributes at each individual position when thegradation pattern is overlapped onto the recording surface;

a record pattern preparing step of determining an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on the first original image for each unit region to whichthe first record attribute was assigned, and determining an interferencefringe pattern, a diffraction grating pattern, or a scattering structurepattern based on the second original image for each unit region to whichthe second record attribute was assigned to prepare data indicating apredetermined record pattern to be formed on the recording surface; and

a medium forming step of forming the record pattern on a physicalmedium.

(5) The fifth feature of the present invention resides in a hologramrecording medium preparing method according to the fourth feature,wherein

the attribute assigning step in turn comprises:

a reference setting step of defining a distance reference line on aplane containing the recording surface;

a distribution factor setting step of defining a distribution factorf(x) as a function of a distance x that takes on values in a range of0≦f(x)≦1; and

an attribute determining step of determining a record attribute to beassigned to each unit region in a manner such that, to unit regions,positioned on or near a positioning line, which is parallel to and is ata position separated from the distance reference line by just a distancex, the first record attribute is assigned at a proportion of f(x) andthe second record attribute is assigned at a proportion of “1−f(x)” orless.

(6) The sixth feature of the present invention resides in a hologramrecording medium preparing method according to the fourth feature,wherein

the attribute assigning step in turn comprises:

a reference setting step of defining a distance reference point on aplane containing the recording surface;

a distribution factor setting step of defining a distribution factorf(x) as a function of a distance x that takes on values in a range of0≦f(x)≦1; and

an attribute determining step of determining a record attribute to beassigned to each unit region in a manner such that, to unit regions,positioned on or near a positioning line, which is defined as acircumference of a circle of a radius x that is centered about thedistance reference point, the first record attribute is assigned at aproportion of f(x) and the second record attribute is assigned at aproportion of “1−f(x)” or less.

(7) The seventh feature of the present invention resides in a hologramrecording medium preparing method according to the fourth feature,wherein

the attribute assigning step in turn comprises:

a reference setting step of defining an angle reference point and anangle reference line, passing through the angle reference point, on aplane containing the recording surface;

a distribution factor setting step of defining a distribution factorf(x) as a function of an angle x that takes on values in a range of0≦f(x)≦1; and

an attribute determining step of determining a record attribute to beassigned to each unit region in a manner such that, to unit regions,positioned on or near a positioning line, which passes through the anglereference point and is inclined by just an angle x with respect to theangle reference line, the first record attribute is assigned at aproportion of f(x) and the second record attribute is assigned at aproportion of “1−f(x)” or less.

(8) The eighth feature of the present invention resides in a hologramrecording medium preparing method according to the fifth to seventhfeatures, wherein

in the distribution factor setting step, a monotonously increasingfunction or a monotonously decreasing function is used as thedistribution factor f(x).

(9) The ninth feature of the present invention resides in a hologramrecording medium preparing method according to the fifth to seventhfeatures, wherein

in the attribute determining step, for a plurality N of unit regionspositioned on a same positioning line, a process of using random numbersto assign the first record attribute to N×f(x) unit regions and assignthe second record attribute to the remaining unit regions is performed.

(10) The tenth feature of the present invention resides in a hologramrecording medium preparing method according to the fifth to seventhfeatures, wherein

in the attribute determining step, an integer ratio α:β thatapproximates “f(x)”:“1−f(x)” is determined for each individualpositioning line, and for (α+β) successive unit regions among aplurality of unit regions positioned on a single positioning line, thefirst record attribute is assigned to α unit regions and the secondrecord attribute is assigned to β unit regions.

(11) The eleventh feature of the present invention resides in a hologramrecording medium preparing method according to the fourth feature,wherein

the attribute assigning step in turn comprises:

a reference setting step of defining a two-dimensional XY coordinatesystem on a plane containing the recording surface;

a distribution factor setting step of defining a distribution factorf(x, y) as a function of two variables x and y of the two-dimensional XYcoordinate system that takes on values in a range of 0≦f(x, y)≦1; and

an attribute determining step of determining position coordinates (x, y)for the respective unit regions and determining record attributes to beassigned to the respective unit regions in a manner such that the firstrecord attribute is assigned at a proportion of f(x, y) and the secondrecord attribute is assigned at a proportion of “1−f(x, y)” or less.

(12) The twelfth feature of the present invention resides in a hologramrecording medium preparing method according to the fourth feature,wherein

the attribute assigning step in turn comprises:

a distribution factor setting step of preparing a table that defines adistribution factor f, taking on values in a range of 0≦f≦1, for eachindividual unit region; and

an attribute determining step of determining record attributes to beassigned to the respective unit regions in a manner such that with eachunit region, the first record attribute is assigned at a proportion of fdefined by the table and the second record attribute is assigned at aproportion of “1−f” or less.

(13) The thirteenth feature of the present invention resides in ahologram recording medium preparing method according to the first totwelfth features, wherein

in the unit region defining step, a plurality of unit regions, having asame size and same rectangular shape and arrayed in a form of atwo-dimensional matrix, are defined.

(14) The fourteenth feature of the present invention resides in ahologram recording medium preparing method according to the thirteenthfeature, wherein

in the attribute determining step, a dithering process using a dithermask, comprising an array adapted to the matrix of unit regions, isperformed to determine record attributes of the respective individualunit regions.

(15) The fifteenth feature of the present invention resides in ahologram recording medium preparing method according to the thirteenthfeature, wherein

in the attribute determining step, a process using an error diffusionmethod is performed to determine record attributes of the respectiveindividual unit regions.

(16) The sixteenth feature of the present invention resides in ahologram recording medium preparing method according to the first tofifteenth features, wherein

in the original image preparing step, digital data, expressing atwo-dimensional image or a three-dimensional image, are prepared as anoriginal image.

(17) The seventeenth feature of the present invention resides in ahologram recording medium preparing method according to the third tosixteenth features, wherein

in the original image preparing step, an empty image without an actualentity is prepared as one of the original images and no patternwhatsoever is formed for unit regions that have been assigned a recordattribute of the empty image.

(18) The eighteenth feature of the present invention resides in ahologram recording medium preparing method according to the first toseventeenth features, wherein

in determining an interference fringe pattern based on an original imagefor a unit region in the record pattern preparing step, an originalimage and the recording surface are positioned in a three-dimensionalspace, a predetermined reference light is defined, and the interferencefringe pattern, formed in the unit region by an object light from theoriginal image and the reference light, is determined by computation.

(19) The nineteenth feature of the present invention resides in ahologram recording medium preparing method according to the first toseventeenth features, wherein

in determining a diffraction grating pattern or a scattering structurepattern based on an original image for a unit region in the recordpattern preparing step, one or a plurality of pixels are defined in theunit region, a corresponding pixel or pixels on the original image is orare determined for the defined pixel or pixels, and a diffractiongrating pattern or a scattering structure pattern in each individualdefined pixel is determined based on a pixel value of the correspondingpixel.

(20) The twentieth feature of the present invention resides in ahologram recording medium preparing method according to the fourth tofifteenth features, wherein

in the record pattern preparing step, the first original image, thesecond original image, and the recording surface are positioned in athree-dimensional space, a predetermined reference light is defined, aninterference fringe pattern of object light from the first originalimage and the reference light is determined by computation for each unitregion, to which the first record attribute is assigned, and aninterference fringe pattern of object light from the second originalimage and the reference light is determined by computation for each unitregion, to which the second record attribute is assigned.

(21) The twenty-first feature of the present invention resides in ahologram recording medium preparing method according to the fourth tofifteenth features, wherein

in the record pattern preparing step, the first original image and therecording surface are positioned in a three-dimensional space, apredetermined reference light is defined, and an interference fringepattern of object light from the first original image and the referencelight is determined by computation for each unit region to which thefirst record attribute is assigned, and for each unit region to whichthe second record attribute is assigned, one or a plurality of pixels isor are defined in a unit region, a corresponding pixel or pixels on thesecond original image is or are determined for the defined pixel orpixels, and a diffraction grating pattern or a scattering structurepattern in each individual defined pixel is determined based on a pixelvalue of the corresponding pixel.

(22) The twenty-second feature of the present invention resides in ahologram recording medium preparing method according to the fourth tofifteenth features, wherein

in the record pattern preparing step, for each unit region to which thefirst record attribute is assigned, one or a plurality of pixels is orare defined in the unit region, a corresponding pixel or pixels on thefirst original image is or are determined for the defined pixel orpixels, and a diffraction grating pattern or a scattering structurepattern in each individual defined pixel is determined based on a pixelvalue of the corresponding pixel, and for each unit region to which thesecond record attribute is assigned, one or a plurality of pixels is orare defined in the unit region, a corresponding pixel or pixels on thesecond original image is or are determined for the defined pixel orpixels, and a diffraction grating pattern or a scattering structurepattern in each individual defined pixel is determined based on a pixelvalue of the corresponding pixel.

(23) The twenty-third feature of the present invention resides in ahologram recording medium preparing method according to the first totwenty-second features, wherein

a size of each unit region is set to a size, by which the presence ofeach individual unit region cannot be recognized by a naked eye.

(24) The twenty-fourth feature of the present invention resides in acomputer program, having a function of making a computer execute theprocess of the attribute assigning step and the process of the recordpattern preparing step of hologram recording medium preparing methodaccording to the first to twenty-third features;

on the basis of digital data, expressing an original image prepared inthe original image preparing step of the preparing method according tothe first to twenty-third features, and digital data, expressing theunit regions defined in the unit region defining step of the preparingmethod according to the first to twenty-third features.

(25) The twenty-fifth feature of the present invention resides in ahologram recording medium, prepared by the preparing method according tothe first to twenty-third features.

(26) The twenty-sixth feature of the present invention resides in ahologram recording medium preparing device comprising:

an original image storage unit, storing, as data, a plurality M oforiginal images to be recorded;

a unit region defining unit, defining and positioning a plurality ofunit regions, each having an adequate area for recording interferencefringes of visible light, on a hologram recording surface;

an attribute assigning unit, assigning a record attribute to each unitregion according to appearance probabilities of respective recordattributes at each individual position when a gradation pattern for eachof the M original images, which expresses that an appearance probabilityof a record attribute corresponding to an original image changesgradually in space, is overlapped onto the recording surface; and

a record pattern preparing unit, determining, for each individual unitregion, an interference fringe pattern, a diffraction grating pattern,or a scattering structure pattern based on a specific original imagecorresponding to an assigned record attribute to prepare data indicatinga predetermined record pattern to be formed on the recording surface.

(27) The twenty-seventh feature of the present invention resides in ahologram recording medium, having a recording surface, on which aplurality of unit regions, each having an adequate area for recordinginterference fringes of visible light, are defined, and wherein imageinformation concerning one original image, among a plurality M oforiginal images, is recorded as an interference fringe pattern, adiffraction grating pattern, or a scattering structure pattern in eachunit region, and appearance probabilities of unit regions, in whichimage information concerning the respective original images arerecorded, change in space.

(28) The twenty-eighth feature of the present invention resides in ahologram recording medium, having a recording surface, on which aplurality of unit regions, each having an adequate area for recordinginterference fringes of visible light, are defined, and wherein eitherimage information concerning a first original image or image informationconcerning a second original image is recorded as an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern in each unit region, and an appearance probability of unitregions, having a first record attribute and in which image informationconcerning the first original image is recorded, and an appearanceprobability of unit regions, having a second record attribute and inwhich image information concerning the second original image isrecorded, gradually change in space,

With the hologram recording medium preparing method according to thepresent invention, gradated motif expression is enabled to enable goodblending to be secured among the motifs. In particular, even when aplurality of motifs are recorded upon being positioned adjacently,because gradation can be applied to the boundary portions, a designexpression such that the plurality of motifs are blended at the boundaryportions is made possible.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of an example of a conventional hologram recordingmedium, in which two motifs are expressed upon simply being positionedadjacently.

FIG. 2 is a plan view of an example of a hologram recording medium, withwhich a gradated expression is applied to boundary portions of twomotifs by a method according to the present invention.

FIG. 3 is a flowchart of a hologram recording medium preparing methodaccording to a basic embodiment of the present invention.

FIG. 4A and FIG. 4B are front views of two original images that are tobe recorded by the method according to the present invention, and FIG.4C is a front view of a recording surface, on which a plurality of unitregions are defined.

FIG. 5 is a graph of a distribution factor function of a gradationpattern used to prepare the hologram recording medium shown in FIG. 2.

FIG. 6 is a plan view showing the gradation pattern, defined by thedistribution factor function f(x) shown in FIG. 5, as a gray densitypattern.

FIG. 7 is a plan view of record attributes assigned to the respectiveunit regions by overlapping the gradation pattern, shown in FIG. 6, ontothe recording surface, shown in FIG. 4C.

FIG. 8 is a perspective view of principles of recording interferencefringe patterns by positioning the two original images, shown in FIGS.4A and 4B, and the recording surface, shown in FIG. 4C, in athree-dimensional space.

FIG. 9 is a plan view of an example of assigning record attributes tounit regions positioned on a specific positioning line Lx on therecording surface Rec.

FIG. 10 is a plan view showing another method for defining thepositioning line Lx on the recording surface Rec.

FIG. 11 is a plan view showing an example of a spherically changinggradation pattern as a gray density pattern.

FIG. 12 is a plan view of a method for defining a positioning line Lxalong a circumference by overlapping the gradation pattern shown in FIG.11 onto the recording surface Rec.

FIG. 13 is a plan view showing an example of a gradation pattern, whichchanges in a rotation direction, as a gray density pattern.

FIG. 14 is a plan view of a method for defining a positioning line Lxalong a radius by overlapping the gradation pattern shown in FIG. 13onto the recording surface Rec.

FIG. 15 is a plan view of an example of defining a gradation pattern onthe recording surface Rec using a distribution factor f(x, y) expressedby a two-dimensional function.

FIG. 16 is a plan view of an example of a table constituted of atwo-dimensional array for defining distribution factors f.

FIG. 17 is a plan view of a method for determining record attributes ofrespective individual unit regions by performing a dithering processusing a dither mask.

FIG. 18 is a plan view of the record attributes of the respectiveindividual unit regions determined by the method shown in FIG. 17.

FIG. 19 is a perspective view of a method for restricting spread anglesof an object light in determining an interference fringe pattern on therecording surface Rec.

FIG. 20A shows a plan view of a recorded motif using diffraction gratingpatterns and FIG. 20B shows a plan view of a recording surface Rec.

FIG. 21 is a plan view of a state in which a motif has been recorded onthe recording surface Rec by using diffraction grating patterns.

FIG. 22 is an enlarged plan view of a diffraction grating pattern formedin a pixel P1 shown in FIG. 21.

FIG. 23 is a block diagram of a basic arrangement of a hologramrecording medium preparing device according to the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention shall now be described based on the illustratedembodiments.

Section 1. Basic Embodiment

First, basic concepts of the present invention shall be described. FIG.1 is a plan view of an example of a conventional hologram recordingmedium, in which two motifs are expressed upon being positionedadjacently. A boundary line C is set at a position at approximately halfof a transverse width L of the hologram recording medium, a motif of anautomobile is recorded in a left half, and a motif of three-dimensionalcharacters of “PAT” is recorded in a right half. This recording mediumis thus formed by positioning the medium of the left half, in which theautomobile motif is recorded, and the medium of the right half, in whichthe three-dimensional character motif is recorded, next to each otheracross the boundary line C. Such a recording medium can be prepared by ageneral, conventional hologram recording medium preparing method.Conventionally, such a method for recording two motifs by partitioningby a contour line C is employed, for example, in a case where a companylogo mark and a character string indicating a company name are to berecorded respectively as motifs positioned next to each other at leftand right sides.

However, as mentioned above, when a plurality of motifs are simplypositioned adjacently in this manner, the motifs do not blend well witheach other and the resulting hologram lacks design quality. The presentinvention proposes a new method for improving the design quality bysecuring blending of recorded motifs by carrying out a gradated motifexpression.

FIG. 2 is a plan view of an example of a hologram recording medium, withwhich a gradated expression is applied to boundary portions of twomotifs by a method according to the present invention. Although in therecording medium shown in FIG. 2, the automobile motif and thethree-dimensional character motif of “PAT” are recorded as in therecording medium shown in FIG. 1, the boundary between the two motifs isnot clear and the motifs are expressed so as to blend near a centralportion. This is a result of applying a gradated expression to a rightportion of the automobile motif and to a left portion of thethree-dimensional character motif. From a comparison with theconventional recording medium, shown in FIG. 1, it can be understoodthat with the recording medium according to the present invention, shownin FIG. 2, the two motifs are mixed as if dissolving into each other,and blending of both motifs is secured so that a design that is integralas a whole is realized.

With general images (images that are not recorded as interference fringepatterns or diffraction grating patterns), image processing arts bywhich two motifs are gradually blended by applying gradation as shown inFIG. 2 have been practiced of old. To blend two pieces of digital imagedata, a method called α-blending is generally used. In this method, whentwo images are overlapped, the pixel value of a pixel at a position ofmutual overlap is determined by synthesis at a ratio of: “α:(1−α)”(where 0≦α≦1). For example, to synthesize a pixel value Pa of an image Aand a pixel value Pb of an image B to determine a new, synthesized pixelvalue, the new pixel value Pc is determined by the formula:“Pc=α−Pa+(1−α)·Pb.” By gradually changing the value of a in space, twoimages can be blended as shown in FIG. 2.

However, in the case of a hologram recording medium, this α-blendingmethod for a general, planar image cannot be applied as it is. This isbecause whereas a general planar image is observed as a distribution ofdensities or luminances of the respective individual pixels, in the caseof hologram recording medium, a reproduction image is observed due todiffracted light that results from diffraction by an interference fringepattern or a diffraction grating pattern recorded on the medium andpropagates toward a viewpoint direction. Even if, in actually recordinga synthetic image of an image A and an image B as a hologram onto amedium, a method of simply overlapping the interference fringe patternor the diffraction grating pattern of both images is employed, recordingcannot be performed in a manner such that a clear reproduction image forpractical purposes is obtained.

The present invention proposes a new method that enables a gradatedmotif expression, such as that shown in FIG. 2, to be carried out on ahologram recording medium. A procedure of a hologram recording mediumpreparing method according to a basic embodiment of the presentinvention shall now be described with reference to a flowchart of FIG.3. The basic procedure shown in FIG. 3 is constituted of an originalimage preparing step (S10), a unit region defining step (S20), anattribute assigning step (S30), a record pattern preparing step (S40),and a medium forming step (S50). Specific processing details of theserespective steps shall now be described in line with an example ofpreparing the recording medium shown in FIG. 2.

As mentioned above, two types of motifs are recorded in the recordingmedium shown in FIG. 2. Thus, in the “original image preparing step” ofstep S10, a first original image and a second original image that are tobe recorded are prepared as data. The two pieces of original image datacorrespond to the respective motifs. FIG. 4A shows a first originalimage Pic(A) for expressing the automobile motif (although a front viewis shown in the figure, the image is actually a three-dimensional imagewith the shape of an automobile), and FIG. 4N shows a second originalimage Pic(B) for expressing the three-dimensional characters of “PAT.”These pieces of data that express the original images are both preparedas digital data. Here, it shall be deemed that the respective originalimages Pic(A) and Pic(B) are prepared as data of three-dimensionalobjects defined in a three-dimensional XYZ coordinate system. Although athree-dimensional object is normally expressed as data of a formexpressing a collection of a plurality of polygons, the data form of theoriginal image data prepared here may be any data form.

The “unit region defining step” of step S20 is carried out next. Here, aprocess of defining and positioning a plurality of unit regions, eachhaving an adequate area for recording visible light interferencefringes, on a hologram recording surface is performed. FIG. 4C shows anexample of a plurality of unit regions U defined on a recording surfaceRec. In the illustrated example, a plurality of unit regions, arrayed ina two-dimensional matrix form and each having the same size and samerectangular shape (a square shape in the present example), are defined.Although the individual unit regions do not necessarily have to be thesame in size and shape, for practical use, it is preferable to arrayunit regions of the same size and same shape because the process is thensimplified. Also, although a hexagon, triangle, etc., may be employed asthe shape of each unit region, for practical use, it is preferable toarray unit regions of rectangular shape in a two-dimensional matrix formas in the illustrated example.

As shall be described later, an independent interference fringe patternor diffraction grating pattern is to be recorded in the interior of eachindividual unit region U, and thus, during observation of the medium,visible light must be made to undergo a predetermined diffractionphenomenon in each individual unit region U and propagate toward aviewpoint position. Each unit region must thus have an adequate area forrecording interference fringes of visible light.

In general, it is deemed that at least approximately five fringes arerequired to give rise to an optically adequate diffraction phenomenon.Here, if the wavelength of red light at the long wavelength side is 650nm and the pitch of fringes suitable for diffraction of red light is thesame 650 nm as the wavelength, a width of 650 nm×5=3.25 μm is needed toposition five fringes. Thus, in regard to the size of a unit region U, aregion of at least 3.25 μm×3.25 μm is necessary.

Meanwhile, as shall be described later, information of mutuallydifferent original images must be recorded in adjacent unit regions U.Thus, if each individual unit region U becomes large enough to berecognized by the naked eye, boundaries between unit regions will beobserved by the naked eye at portions of adjacent unit regions in whichthe information of mutually different original images are recorded andmake the observed image look awkward. It is thus preferable to limit themaximum size of each individual unit region U to a size such that thepresence of each individual unit region U cannot be recognized by thenaked eye. Specifically, it is deemed that even if an array of aplurality of cells is present, if the dimension of each cell is no morethan 300 μm, it is significantly difficult to recognize the cell arrayby the resolution of the naked eye. The size of each unit region Udefined in the present invention is thus preferably set to 300 μm at themost.

The size of each unit region U defined in step S20 is thus preferablyset in the range of 3.25 μm to 300 μm. With the embodiment describedhere, the unit regions U of square shape are positioned in atwo-dimensional matrix form as shown in FIG. 4C, and a single unitregion U is set as a square of 20 μm×20 μm. Although for the sake ofillustration, a unit region array of 8 rows and 16 columns is shown inFIG. 4C, an array, formed of a larger number of unit regions, is definedin actuality. By defining unit regions of such size, each unit region ismade to have an adequate area for recording interference fringes ofvisible light and yet individual unit regions are not recognized bynaked eye observation.

The hologram recording surface, on which the unit regions U are definedin step S20, is merely a conceptual, geometrical recording surface atthis step. Of the respective steps shown in FIG. 3, the processes up tothat of the “record pattern preparing step” of step S40 are actuallyprocesses carried out inside a computer. Therefore, the unit regiondefinition process in step S20 is also actually a process of setting upindividual unit regions on a conceptual recording surface on a computer.

The “attribute assigning step” of step S30 is carried out next. Anexample, in which this step S30 is constituted of procedures of thethree steps of a “reference setting step” of S31, a “distribution factorsetting step” of step S32, and an “attribute determining step” of stepS33, is shown, and these detailed procedures shall be described inSection 2.

Of the basic processes carried out in the “attribute assigning step” ofstep S30, first, a gradation pattern, which expresses that an appearanceprobability of a first record attribute and an appearance probability ofa second record attribute change gradually in space, is defined. Thisshall now be described in line with a specific example. FIG. 5 is agraph of a distribution factor function of a gradation pattern that isused to prepare the hologram recording medium shown in FIG. 2. Theabscissa of the graph indicates a distance x and the ordinate indicatesa distribution factor f(x). Here, the distribution factor f(x) is afunction of x that takes on values in a range of 0≦f(x)≦1, and in thepresent example, f(x)=1 in a distance range of x≦L/4, f(x)=0 in adistance range of x≧3L/4, and f(x) is a function that graduallydecreases monotonously with x in a distance range of L/4<x<3L/4. L isthe transverse width of the recording medium as shown in FIG. 2, and atthe position of x=2L/4 (central position in the left/right direction inFIG. 2), f(x)=0.5.

FIG. 6 is a plan view showing the gradation pattern, defined by thedistribution factor function f(x) shown in FIG. 5, as a gray densitypattern. That is, f(x)=1 is expressed by white, f(x)=0 is expressed byblack, and intermediate values between 1 and 0 are expressed by gray ofpredetermined densities. The recording medium shown in FIG. 2corresponds to overlappingly recording the first original image Pic(A),shown in FIG. 4A, and the second original image Pic(B), shown in FIG.4B, according to the gradation pattern density information shown in FIG.6. That is, the recording medium shown in FIG. 2 corresponds toblendingly recording the two original images in a manner such that thestronger the white color in the gradation pattern, the higher the weightof the first original image Pic(A), and the stronger the black color inthe gradation pattern, the higher the weight of the second originalimage Pic(B).

As mentioned above, with general images (images that are not recorded asinterference fringe patterns or diffraction grating patterns), theblending of two original images according to a gradation pattern, suchas shown in FIG. 6, can be performed by the known process called“α-blending.” That is, if the distribution factor f(x), shown in thegraph of FIG. 5, is used as it is as the α value, a pixel value Pc of apixel at a predetermined position of an image C obtained by blending animage A and an image B is determined by the formula:“Pc=f(x)·Pa+(1−f(x))·Pb,” based on a pixel value Pa of the image A and apixel value Pb of the image B at the corresponding position.

However, as mentioned above, in the case of a hologram recording medium,the α-blending method for such general images cannot be applied as itis. Thus, with the present invention, blending according to thegradation pattern is realized based on the following basic concept.

That is, firstly, in a single unit region, just one of eitherinformation concerning the first original image Pic(A) or informationconcerning the second original image Pic(B) is recorded selectively.Although in the case of α-blending of general images, processing isperformed with the idea of blending the pixel values of the two imagesto be blended, in the present invention, a method of blending pixelvalues is not employed and only information concerning one of either ofthe original images is recorded in a single unit region. In a hologramrecording medium, information on original images are recorded asinterference fringe patterns or diffraction grating patterns, and byrecording an interference fringe pattern or a diffraction gratingpattern concerning just one of the original images in a single unitregion, the diffraction phenomenon can be made to occur efficiently anda clear reproduction image can be obtained.

Secondly, the gradation pattern information is reflected in theselection between recording information concerning the first originalimage Pic(A) and recording information concerning the second originalimage Pic(B) in each individual unit region. For example, if selectionbetween the first original image Pic(A) and the second original imagePic(B) is to be performed using the gradation pattern shown in FIG. 6, aprocess of overlapping the gradation pattern of FIG. 6 onto therecording surface Rec, on which the array of the respective unit regionsU, shown in FIG. 4C, has been defined, and selecting one of the twooriginal images according to the density of the gradation pattern oneach unit region U is performed.

Because the gradation pattern of FIG. 6 corresponds to the distributionfactor function f(x) of FIG. 5, by overlapping this gradation patternonto the recording surface Rec, a predetermined distribution factor f(x)can be defined for each individual unit region U. For example, bydefining a positional reference point for each unit region(specifically, for example, a central point of each unit region may bedefined as the positional reference point), the predetermineddistribution factor f(x) can be defined based on the x-coordinate valueof this positional reference point. A process of selecting one of theoriginal images in a manner such that the first original image Pic(A) isselected at a probability of the distribution factor f(x) and the secondoriginal image Pic(B) is selected at a probability of 1−f(x) is thenperformed.

With the present invention, “selection of a specific original image tobe recorded in a specific unit region” is expressed as “assigning arecord attribute of a specific original image to a specific unitregion.” With the above example, a first record attribute “A” isassigned to a unit region, for which the first original image Pic(A) hasbeen selected as the original image to be recorded, and a second recordattribute “B” is assigned to a unit region, for which the secondoriginal image Pic(B) has been selected as the original image to berecorded.

FIG. 7 is a plan view of the record attributes assigned to therespective unit regions by overlapping the gradation pattern, shown inFIG. 6, onto the recording surface, shown in FIG. 4C. Each individualcell of square shape, shown in FIG. 7, corresponds to an individual unitregion U, shown in FIG. 4C, and the character “A” or “B” that isindicated in each cell indicates the record attribute assigned to thecorresponding unit region. Because with the distribution factor functionf(x), shown in FIG. 5, f(x)=1 in the range of 0≦x≦L/4, the recordattribute “A” is assigned at a probability of 100% to the unit regions(of column numbers 1 to 4) positioned in the range of 0≦x≦L/4 in FIG. 7.Likewise, because with the distribution factor function f(x), shown inFIG. 5, f(x)=0 in the range of 3L/4≦x≦L, the record attribute “B” isassigned at a probability of 100% to the unit regions (of column numbers13 to 16) positioned in the range of 3L/4≦x≦L in FIG. 7. In theintermediate range of L/4<x<3L/4 (column numbers 5 to 12), theappearance probability of the record attribute “A” becomes higher towardthe left side and the appearance probability of the record attribute “B”becomes higher toward the right side.

The distribution factor function f(x), shown in FIG. 5, and thegradation pattern, shown in FIG. 6, thus indicate the appearanceprobability of each record attribute at each individual position.Because in the example illustrated here, just one of either the recordattribute “A” or the record attribute “B” is always assigned to eachunit region, the assigning of the record attribute “B” is acomplementary event with respect to the assigning of the recordattribute “A.” The sum of the appearance probability of the recordattribute “A” and the appearance probability of the record attribute “B”is thus always 1. Put in another way, although the distribution factorfunction f(x) in itself expresses the appearance probability of therecord attribute “A,” it also indirectly expresses the appearanceprobability of the record attribute “B” in the form of “1−f(x).”

Thus, in the “attribute assigning step” of step S30 of FIG. 3, aprocess, of defining a gradation pattern, which expresses that theappearance probability of the first record attribute “A” and theappearance probability of the second record attribute “B” graduallychange in space, and assigning one of either the first record attribute“A” or the second record attribute “B” to each unit region according tothe “appearance probability of each record attribute” at each individualposition when the gradation pattern is overlapped onto the recordingsurface Rec, is performed.

Although in the embodiment described here, one of either the firstrecord attribute “A” or the second record attribute “B” is alwaysassigned to each unit region and it is deemed that there are no unitregions to which a record attribute is not assigned, unit regions, towhich neither record attribute is assigned, may be set up in some cases.That is, in the embodiment described here, the appearance probability ofthe record attribute “A” is defined by the distribution factor functionf(x) and the appearance probability of the record attribute “B” is alsoindirectly defined in the form of “1−f(x).” Although this definition ispremised on the assigning of the record attribute “B” being acomplementary event of the assigning of the record attribute “A,” asetting in which the appearance probability of the record attribute “B”is made less than “1−f(x)” is also possible. With such a setting, thesum of the appearance probability of the record attribute “A” and theappearance probability of the record attribute “B” is not 1.

For example, by defining the appearance probability of the recordattribute “A” by a distribution factor function fa(x), defining theappearance probability of the record attribute “B” by a distributionfactor function fb(x), and setting these functions so thatfa(x)+fb(x)<1, unit regions, to which neither record attribute isassigned, will appear (with the appearance probability being1−fa(x)−fb(x)). In putting the present invention into practice, suchunit regions, to which neither record attribute is assigned, may bepresent.

When the process of assigning attributes to the respective unit regionis thus completed, the “record pattern preparing step” of step S40,shown in the flowchart of FIG. 3, is executed. In this step, a process,of determining an interference fringe pattern or diffraction gratingpattern, based on the first original image Pic(A), for each unit regionto which the first record attribute “A” was assigned, determining aninterference fringe pattern or diffraction grating pattern, based on thesecond original image Pic(B), for each unit region to which the secondrecord attribute “B” was assigned, and finally preparing data indicatinga predetermined record pattern to be formed on the recording surfaceRec, is executed.

FIG. 8 is a perspective view of principles of recording interferencefringe patterns onto the recording surface Rec by positioning the twooriginal images Pic(A) and Pic(B), shown in FIGS. 4A and 4B, and therecording surface Rec, shown in FIG. 4C, in a three-dimensional space.Because this process is actually executed as a simulation computation ofoptical phenomena on a computer, the original images Pic(A) and Pic(b)and the recording surface Rec are virtual objects positioned in athree-dimensional space on a computer.

Specifically, as shown in the figure, a predetermined reference light Ris set in addition to the original images Pic(A) and Pic(B) and therecording surface Rec, and interference fringe patterns formed atrespective portions of the recording surface Rec by object light emittedfrom the original images Pic(A) and. Pic(B) and the reference light Rare determined by computation by the computer. Although in theillustrated example, the reference light R is set in common for theoriginal images Pic(A) and Pic(B), a reference light Ra for recordingthe original images Pic(A) and a reference light Rb for recording theoriginal images Pic(B) may instead be set separately. Because such aninterference fringe pattern computing method is a general method forcomputer holograms as disclosed, for example, in the abovementionedPatent Documents 1 to 3, detailed description of the computing methoditself shall be omitted.

An important characteristic of the “record pattern preparing step” ofstep S40 of the present invention is that the original image to berecorded differs according to each individual unit region defined on therecording surface Rec. On the recording surface Rec shown in FIG. 8, aplurality of unit regions are defined (the unit region defining step ofstep S20) as shown in FIG. 4C, and each individual unit region isassigned with a predetermined record attribute (the attribute assigningstep of step S30) as shown in FIG. 7. For example, the record attribute“A” is assigned to a unit region Ua on the recording surface Rec, shownin FIG. 8, and the record attribute “B” is assigned to a unit region Ub.Here, in computing and recording the interference fringe patterns, onlyan interference fringe pattern based on the first original image Pic(A)is recorded in the unit region Ua, to which the record attribute “A” isassigned, and only an interference fringe pattern based on the secondoriginal image Pic(B) is recorded in the unit region Ub, to which therecord attribute “B” is assigned.

That is, an interference fringe pattern of the object light from thefirst original image Pic(A) and the reference light R is recorded in theunit region Ua, and in this process, the object light from the secondoriginal image Pic(B) is completely ignored. Likewise, an interferencefringe pattern of the object light from the second original image Pic(B)and the reference light R is recorded in the unit region Ub, and in thisprocess, the object light from the first original image Pic(A) iscompletely ignored. Because in a computer hologram method, theinterference fringe patterns are determined by computation, such aprocess of determining an interference fringe pattern upon selecting theobject light can be carried out freely by a program.

A predetermined record pattern is thus prepared on the recording surfaceRec by the “record pattern preparing step” of step S40, and the objectto be recorded according to the record pattern differs according to eachindividual unit region. That is, with the respective unit regions shownin FIG. 7, interference fringe patterns concerning the first originalimage Pic(A) are recorded in the unit regions to which the recordattribute “A” is assigned, and interference fringe patterns concerningthe second original image Pic(B) are recorded in the unit regions towhich the record attribute “B” is assigned.

Because only an interference fringe pattern concerning a single originalimage is recorded in a single unit region, diffracted light that enablesformation of a clear reproduction image is obtained from each unitregion during observation. Also, because, on the recording surface Rec,the appearance probability of a unit region to which record attribute“A” has been assigned and the appearance probability of a unit region towhich record attribute “B” has been assigned is in accordance with thegradation pattern shown in FIG. 6, when the recording surface Rec isobserved as a whole, a motif according to the original image Pic(A) anda motif according to the original image Pic(B) are respectivelyexpressed in gradated states and the effect of blending of the boundaryportions of the two motifs is obtained as show in FIG. 2.

The mode of observation of the two motifs that are reproduced by thehologram recording medium prepared by the preparing method according tothe present invention differs from the mode of observation of two motifsin a conventional, multi image hologram recording medium. That is,whereas with the conventional multi image hologram recording medium, thefirst motif is observed or the second motif is observed by change of theobservation direction, with the hologram recording medium according tothe present invention, a mode, in which a change from the first motif tothe second motif occurs in space, is observed regardless of theobservation direction.

If in the “attribute assigning step” of step S30, a setting is made suchthat unit regions, to which neither record attribute is assigned, arepresent, neither of the interference fringe patterns of the two imagesis recorded in these unit regions. That is, diffracted light forreproduction is not obtained from these unit regions duringreproduction. Although such unit regions, to which neither recordattribute is assigned, thus do not directly contribute to the object ofdisplaying a reproduction image of the original images, by setting upsuch unit regions at portions, a contribution can be made to the objectof displaying a reproduction image that is gradated as a whole.

Although in the above description, an example of recording two types oforiginal images Pic(A) and Pic(B) next to each other was described, thepresent invention can obviously be used in cases of recording three ormore types of original images next to each other as well. For example,in a case of recording three types of original images Pic(A), Pic(B),and Pic(C) next to each other, a gradation pattern is defined in regardto the respective original images and record attributes “A,” “B,” and“C” are assigned to the respective individual unit regions according tothe appearance probabilities indicated by the gradation pattern. Byemploying such a method, a hologram recording medium, with which, forexample, the motif of the original image Pic(A) gradually changes to themotif of the original image Pic(B) and furthermore changes to the motifof the original image Pic(C) from the left to the right, can beprepared.

In the “medium forming step” of the last step S50 of FIG. 3, a process,of forming a record pattern on a physical medium based on the recordpattern data on the recording surface Rec prepared in step S40, isperformed. Although as mentioned above, the procedures up to step S40are processes that are carried out on a computer, the procedure of stepS50 is a processing procedure of using the record pattern data, preparedby processing on the computer, to form interference fringe patterns ordiffraction grating patterns on an actual, physical medium.Specifically, a process of transmitting the prepared record pattern datato an electron beam drawing equipment, etc., and preparing the physicalmedium is performed. Because specific methods for forming such aphysical hologram recording medium are known arts, detailed descriptionthereof shall be omitted here.

Section 2. Processing in the Attribute Assigning Step

The “attribute assigning step” of step S30 of FIG. 3 shall now bedescribed in more detail. As mentioned above, in this attributeassigning step, a process, of defining a gradation pattern, whichexpresses that the appearance probability of the first record attribute“A” and the appearance probability of the second record attribute “B”change gradually in space, and assigning, to each unit region, one ofeither the first record attribute “A” or the second record attribute “B”or neither record attribute (in a case where unit regions in whichneither pattern is to be recorded are set up) according to theappearance probabilities of the respective record attributes at eachindividual position when the gradation pattern is overlapped onto therecording surface Rec, is performed.

Step S30 in FIG. 3 is constituted of the procedures of the three stepsof the “reference setting step” of S31, the “distribution factor settingstep” of step S32, and the “attribute determining step” of step S33, andthe significances of the procedures of these three steps shall now bedescribed.

FIG. 9 is a plan view of an example of a method for assigningpredetermined record attributes respectively to unit regions U(1) toU(8) positioned on a specific positioning line Lx on the recordingsurface Rec. The recording surface Rec is a rectangular region, and adistance reference line L0 is defined on the left edge thereof. Thisdistance reference line L0 is a line that passes through the origin of acoordinate axis X set in a horizontal direction of the figure, and acoordinate value x along this coordinate axis X indicates the distancefrom the distance reference line L0. For example, a point on thedistance reference line L0 is a point at the distance x=0, and a pointon the right edge of the recording surface Rec is a point at thedistance x=L (L being the transverse width of the recording surfaceRec).

Here, a case where a plurality of unit regions U are defined on therecording surface Rec as shown in FIG. 4C and predetermined recordattributes are to be assigned to the respective individual unit regionsshall be considered. Specifically, one of either the first recordattribute “A” or the second record attribute “B” is to be assigned toeach unit region according to the appearance probabilities of therespective record attributes at each individual position when thegradation pattern, shown in FIG. 6, is overlapped onto the recordingsurface Rec. Here, if the position of each individual unit region isexpressed by the x-coordinate value based on the central point of theunit region, the positions of the illustrated unit regions U(1) to U(8)can be expressed by the x-coordinate value of the positioning line Lxthat passes through the centers of these unit regions. Because thegradation pattern shown in FIG. 6 corresponds to the distribution factorfunction f(x), shown in FIG. 5, the appearance probability of the firstrecord attribute “A” at the position of the positioning line Lx of FIG.9 is expressed by the distribution factor f(x). Here, the distributionfactor f(x) is a function of the distance x and takes on values in arange of 0≦f(x)≦1.

The attributes assigned to the respective unit regions U(1) to U(8) arethus determined so that, in accordance with the distribution factorf(x), the first record attribute “A” is assigned at a proportion of f(x)and the second record attribute “B” is assigned at a proportion of1−f(x). When f(x)=1, the first record attribute “A” is assigned to allunit regions U(1) to U(8) on the positioning line Lx, and when f(x)=0,the second record attribute “B” is assigned to all unit regions U(1) toU(8) on the positioning line Lx.

Although in FIG. 9, the distance reference line L0 is defined on a leftedge of the rectangle constituting the recording surface Rec, thedistance reference line L0 may be defined in any direction at anyposition. For example, FIG. 10 is a plan view showing another example ofa method for defining the positioning line Lx on the recording surfaceRec. In the example of FIG. 10, the distance reference line L0 isdefined outside the recording surface Rec. The distance reference lineL0 may thus be defined at any position and in any direction as long asit is on the plane that contains the recording surface Rec.

Thus, with the embodiments shown in FIGS. 9 and 10, the “referencesetting step” of step 31 that constitutes the step S30 of FIG. 3 is astep of defining the distance reference line L0 on the plane containingthe recording surface Rec, the “distribution factor setting step” ofstep S32 is a step of defining the distribution factor f(x), which, as afunction of the distance x, takes on values in the range of 0≦f(x)≦1,and the “attribute determining step” of step S33 is a step ofdetermining the record attribute to be assigned to each unit region in amanner such that, to the unit regions, positioned on the positioningline Lx, which is parallel to and separated from the distance referenceline L0 by just the distance x, the first record attribute “A” isassigned at a proportion of f(x) and the second record attribute “B” isassigned at a proportion of “1−f(x).”

Because each individual unit region is a region having an area, the“distance from the distance reference line L0” does not necessarily haveto be defined strictly. For example, although with the example shown inFIG. 9, the distance between the unit regions U(1) to U(8) and thedistance reference line L0 is defined by the position x of thepositioning line Lx that passes through the central points of these unitregions, the positioning line Lx may instead be defined on left edges,right edges, or any other position of the respective unit regions.

Also, attribute determination may be performed so that, in assigning theattributes to the total of 16 unit regions positioned in the 8th and 9thcolumns in FIG. 7, the x-coordinate value (x=2L/4) of a positioning lineLx, at a boundary line position between the 8th and 9th columns, is usedto assign the first record attribute “A” at a proportion of f(x) andassign the second record attribute “B” at a proportion of “1−f(x).” Inthis case, attribute assignment is performed using the coordinate valuex of x=2L/4 in common for the 16 unit regions that positioned near thepositioning line Lx, which is parallel to and separated by just thedistance x=2L/4 from the distance reference line L0.

Thus, in the “attribute determining step” of step S33, attributedetermination, such that the first record attribute “A” is assigned at aproportion of f(x) and the second record attribute “B” is assigned at aproportion of “1−f(x),” may be performed not only on unit regionspositioned on the positioning line Lx, which is parallel to andseparated by just the distance x from the distance reference line L0,but also on unit regions positioned near the positioning line Lx.

It is also possible, as described above, to assign neither of the firstrecord attribute “A” and the second record attribute “B” to some of theunit regions and make these regions those in which no pattern is formedin the final stage. In this case, the proportion by which the secondrecord attribute “B” is assigned is set not to the proportion of“1−f(x)” but to a predetermined proportion less than “1−f(x).” Forexample, whereas normally when f(x)=0.7, one of either record attributesis assigned by assigning the first record attribute “A” to 70% of theunit regions of the entirety and assigning the second record attribute“B” to 30% (1−0.7=0.3) of the unit regions of the entirety, if thesecond record attribute “B” is assigned to less than 30% of the entirety(for example, to 20% of the unit regions of the entirety), a portion ofthe unit regions (10% of the unit regions of the entirety in the case ofthe above example) is left without being assigned with either recordattribute.

Thus, by setting the proportion at which the second record attribute “B”is assigned not to the proportion of “1−f(x)” but to the predeterminedproportion less than “1−f(x),” unit regions, to which neither recordattribute is assigned, arise. Although such unit regions, to whichneither record attribute is assigned, do not directly contribute to theobject of displaying a reproduction image of the original images, bysetting up such unit regions at portions, a contribution can be made tothe object of displaying a reproduction image that is gradated as awhole as described above.

Although the distribution factor function f(x), defined in the“distribution factor setting step” of step S32 may be any function, forpractical purposes, the use of a monotonously decreasing function, suchas shown in the graph of FIG. 5, or oppositely a monotonously increasingfunction is preferable. By using a monotonously decreasing function or amonotonously increasing function as the distribution factor functionf(x), a “gradation pattern with unidirectionality in the densitychange,” such as shown in FIG. 6, can be defined to enable a naturalexpression such that a first motif changes gradually to a second motiffrom the left to the right as in the example shown in FIG. 2.

In the “attribute determining step” of step S33, a process of assigningthe first record attribute “A” and the second record attribute “B” atpredetermined proportions to the plurality of unit regions positioned onthe positioning line Lx is performed, and some specific methods forassigning specific record attributes at such predetermined proportionsshall now be described.

In a simplest method for assigning one of either record attributes toeach unit region, random numbers are used to assign the first recordattribute “A” to N×f(x) unit regions among a plurality N of unit regionspositioned on the same positioning line and assign the second recordattribute “B” to the remaining unit regions. Consider, for example, acase in which, for the total of 16 unit regions positioned in the 8thcolumn and 9th column of FIG. 7, the first record attribute “A” is to beassigned at a proportion of the distribution factor f(x)=0.5 and thesecond record attribute “B” is to be assigned to the remaining unitregions. In this case, for each individual unit region, a random numberbetween 0 and 1 is generated, the first record attribute “A” is assignedwhen a random number value of no more than 0.5 is obtained, and thesecond record attribute “B” is assigned when a random number valueexceeding 0.5 is obtained. By doing so, the first record attribute “A”is stochastically assigned to 8 unit regions of the total of 16 unitregions and the second record attribute “B” is assigned to the remaining8 unit regions.

As another method for assigning record attributes, an integer ratio α:βthat approximates “f(x)” “1−f(x)” is determined for each individualpositioning line, and for (α+β) successive unit regions among theplurality of unit regions positioned on a single positioning line, thefirst record attribute “A” is assigned to α unit regions and the secondrecord attribute “B” is assigned to β unit regions. For example, iff(x)=0.24, the integer ratio, 1:3, can be determined as the integerratio α:β that approximates “f(x)”:“1−f(x).” Thus, in this case, thefirst record attribute “A” is assigned to 1 unit region and the secondrecord attribute “B” is assigned to 3 unit regions of 4 successive unitregions among the 8 unit regions U(1) to U(8) positioned on the singlepositioning line Lx shown in FIG. 9. Here, the first record attribute“A” is assigned to the unit region U(1), the second record attribute “B”is assigned to the unit regions U(2) to U(4), the first record attribute“A” is assigned to the unit region U(5), and the second record attribute“B” is assigned to the unit regions U(6) to U(8).

Although with the embodiments described up until now, examples of usinga gradation pattern, with which a density change appears in thehorizontal direction as shown in FIG. 6, were described, variousgradation patterns can be used to assign record attributes with themethod according to the present invention. Some variations of gradationpatterns shall now be described.

FIG. 11 is a plan view showing an example of a spherically changinggradation pattern as a gray density pattern. In this example, a graygradation pattern, with which the density of black increases withincrease of a distance x from a distance reference point Q, set outsidethe pattern, is shown. FIG. 12 is a plan view of a method for defining apositioning line Lx along a circumference by overlapping the gradationpattern shown in FIG. 11 onto the recording surface Rec. Whereas withthe example shown in FIG. 9, the positioning line Lx is a straight line,with the example shown in FIG. 12, an arcuate positioning line Lx isdefined, and predetermined record attributes are assigned in proportionsaccording to the distribution factor f(x) to unit regions positioned onor near the arcuate positioning line Lx.

In using such a gradation pattern, a process of defining the distancereference point Q on the plane containing the recording surface Rec isperformed in the “reference setting step” of step S31, the distributionfactor f(x), which takes on values in the range of 0≦f(x)≦1, is definedas a function of the distance x in the “distribution factor settingstep” of step S32, and, in the “attribute determining step” of step S33,the record attributes to be assigned to the respective unit regions aredetermined so that the first record attribute “A” is assigned at aproportion of f(x) and the second record attribute “B” is assigned at aproportion of “1−f(x)” or less to the unit regions positioned on or nearthe positioning line Lx, defined as the circumference of the circle of aradius x centered about the distance reference point Q.

When a gradation pattern such as that shown in FIG. 11 is used toperform recording concerning two original images, a hologram recordingmedium, with which the first motif gradually changes to the second motifin a spherical manner centered about the distance reference point Q, isobtained.

Meanwhile, FIG. 13 is a plan view showing an example of a gradationpattern, which changes in a rotation direction, as a gray densitypattern. In this example, a gray gradation pattern, with which thedensity of black increases with increase of a clockwise rotation angle xcentered about an angle reference point QQ, set outside the pattern, isshown. FIG. 14 is a plan view of a method for defining a positioningline Lx along a radius by overlapping the gradation pattern shown inFIG. 13 onto the recording surface Rec. Although as with the exampleshown in FIG. 9, the positioning line Lx itself is a straight line inthe example shown in FIG. 14, the positioning line Lx is defined as astraight line passing through the angle reference point QQ and theseparation from an angle reference line LL0 that passes through theangle reference point QQ is defined not as a distance but as an anglevalue x. Predetermined record attributes are assigned in proportionsaccording to the distribution factor f(x) to unit regions positioned onor near the positioning line Lx in this example as well.

In using such a gradation pattern, a process of defining the anglereference point QQ on the plane containing the recording surface Rec anddefining the angle reference line LL0 that passes through the anglereference point QQ is performed in the “reference setting step” of stepS31, the distribution factor f(x), which takes on values in the range of0≦f(x)≦1, is defined as a function of the angle x in the “distributionfactor setting step” of step S32, and, in the “attribute determiningstep” of step S33, the record attributes to be assigned to therespective unit regions are determined so that the first recordattribute “A” is assigned at a proportion of f(x) and the second recordattribute “B” is assigned at a proportion of “1−f(x)” or less to theunit regions positioned on or near the positioning line Lx that passesthrough the angle reference point QQ and is inclined by just the angle xwith respect to the angle reference line LL0.

When a gradation pattern such as that shown in FIG. 13 is used toperform recording concerning two original images, a hologram recordingmedium, with which the first motif gradually changes to the second motifin a fan-like manner with respect to the angle reference line LL0, isobtained.

Though in all of the examples described up until now, a one-dimensionaldistribution factor function f(x) is defined, the distribution factorfunction does not necessarily have to be a one-dimensional function, anda two-dimensional distribution factor function may be defined and usedinstead. FIG. 15 is a plan view of an example of defining a gradationpattern on the recording surface Rec using a distribution factor f(x, y)expressed by a two-dimensional function. As illustrated, atwo-dimensional XY coordinate system, having an origin O is defined onthe plane containing the recording surface Rec to enable the position(for example, a central point or other predetermined reference pointposition) of any unit region U(x, y) on the recording surface Rec to beexpressed by coordinate values (x, y). Meanwhile, a two-dimensionaldistribution factor function f(x, y) is defined to enable association ofa predetermined distribution factor with each position of coordinates(x, y) on the recording surface Rec. A predetermined distribution factorf(x, y) can thereby be associated with an arbitrary unit region U(x, y),and the record attribute to be assigned to the unit region U(x, y) canbe determined based on the distribution factor f(x, y).

In employing such a method for defining a gradation pattern by atwo-dimensional distribution factor function f(x, y), the followingprocesses are performed in the respective steps. First, in the“reference setting step” of step S31, the two-dimensional XY coordinatesystem is defined on the plane containing the recording surface Rec.Although the origin O and the coordinate axes X and Y do not necessarilyhave to be defined on the recording surface Rec, for practical purposes,the setting of the origin O at one corner of the rectangular recordingsurface Rec, the setting of the X-axis at a lower edge of the recordingsurface Rec, and the setting of the Y-axis at the left edge of therecording surface Rec is convenient in terms of performing coordinatecomputations. In the “distribution factor setting step” of step S32 thatfollows, the distribution factor f(x, y), which takes on values in arange of 0≦f(x, y)≦1, is defined as a function of the two variables xand y of the two-dimensional XY coordinate system. That is, thegradation pattern is defined by the two-dimensional distribution factorfunction f(x, y). Then, in the “attribute determining step” of step S33,the position coordinates (x, y) are determined for the respective unitregions and the record attributes to be assigned to the respective unitregions are determined so that the first record attribute “A” isassigned at a proportion of f(x, y) and the second record attribute “B”is assigned at a proportion of “1−f(x, y)” or less.

Any of various functions can be defined by various formulae as thetwo-dimensional distribution factor function f(x, y). Gradation patternsof an extremely high degree of freedom can thus be defined in comparisonto the above-described cases of using a one-dimensional distributionfactor function f(x).

The distribution factors to be used for record attribute assignment donot necessarily have to be defined in the form of a function, and may beprepared in the form of a table instead. FIG. 16 is a plan view of anexample of a table constituted of a two-dimensional array for definingdistribution factors f. This table is constituted of a matrix of thesame size as the unit region array shown in FIG. 4C, and a value of apredetermined distribution factor f is defined for each individual cell.In FIG. 16, f1 to f128 are distribution factor values defined for therespective individual cells. Put in another way, the respectiveindividual cells of this table are in a one-to-one correspondence withthe respective individual unit regions defined on the recording surfaceRec and can serve a function of providing a unique distribution factorto each individual unit region U.

In employing such a method for defining the gradation pattern using atable, the “reference setting step” of step S31 does not have to beperformed. In the “distribution factor setting step” of step S32, atable (a table such as that shown in FIG. 16), which defines thedistribution factor f that takes on values in a range of 0≦f≦1 for eachindividual unit region, is prepared, and in the “attribute determiningstep” of step S33, the record attributes to be assigned to therespective unit regions are determined so that the first recordattribute “A” is assigned at a proportion of f defined by the table andthe second record attribute “B” is assigned at a proportion of “1−f” orless. Gradation patterns of a high degree of freedom can be defined byemploying the method for defining the distribution factor by a table.

In defining a gradation pattern by using a two-dimensional distributionfactor function f(x, y) or a table of distribution factors, a pluralityof unit regions, having the same size and same rectangular shape andarrayed in the form of a two-dimensional matrix, are preferably definedin terms of practical use.

Lastly, two methods that are preferably used in performing the processof assigning attributes to the respective unit regions shall bedescribed. These methods are especially effective in performingattribute assignment based on distribution factors defined using atwo-dimensional function f(x, y) or a two-dimensional table.

In a first method, a dithering process, using a dither mask constitutedof an array adapted to the array of unit regions, is performed in theattribute determining step to determine the record attribute of eachindividual unit region. In general, a dithering process is a method usedfor converting a continuous tone image into a binary image in thetechnical field of printing and is widely used to express an image withcontinuous tone by halftone dots. With the present invention, becausethe process of assigning one of either the first record attribute “A” orthe second record attribute “B” to each individual unit region can besubstituted by a process of converting a continuous tone image to abinary image, the attribute determining step in the present inventioncan be performed by a dithering process. A specific method for assigningone of either the first record attribute “A” or the second recordattribute “B” to each individual unit region by a dithering processshall now be described.

FIG. 17 is a plan view of a method for determining record attributes ofrespective individual unit regions by performing a dithering processusing a dither mask. An example of a dither mask D to be used in thisdithering process is shown in an upper stage of FIG. 17. The actualentity of this dither mask D is a matrix of 4 rows and 4 columns, inwhich numerical values 0 to 15 are positioned at predeterminedpositions. Meanwhile, the recording surface Rec, on which a plurality ofunit regions are defined, is shown in a lower stage of FIG. 17. Here,for the sake of description, frames F1 and F2, indicated by thick linesin the figure, are defined. Both frames F1 and F2 are arrays, in each ofwhich unit regions are arrayed in 4 rows and 4 columns as in the dithermask, and 16 unit regions are contained in a single frame.

Here, in order to simplify the description, a positioning line Lx, suchas that illustrated, shall be considered, a distribution factor fx,defined at the position of the positioning line Lx, shall be deemed tobe such that fx=0.5, and a case of performing the process of assigning,based on the distribution factor fx=0.5, one of either the first recordattribute “A” or the second record attribute “B” to each of the total of32 unit regions in the frames F1 and F2 shall be considered. Here,because the appearance probability of the first record attribute “A” is0.5 and the appearance probability of the second record attribute “B” isalso 0.5, the first record attribute “A” is assigned to 16 unit regionsof the total of 32 unit regions and the second record attribute “B” isassigned to the remaining 16 unit regions stochastically.

As the simplest method for assigning either of the record attributesbased on such specific appearance probabilities, the method of usingrandom numbers was described above. However, with the method of usingrandom numbers, even though the first record attribute “A” is assignedto 16 of the 32 unit regions, the distribution of the unit regions, towhich the record attribute “A” is assigned, is completely random. On theother hand, with the attribute determination method using the ditheringprocess to be described here, the distribution of the unit regions, towhich the record attribute “A” is assigned, can be controlled to somedegree according to the form of distribution of the numerical valuesinside the dither mask D used.

With the example shown in FIG. 17, the record attributes to be assignedto the total of 32 unit regions inside the frame F1 and F2 aredetermined as follows. First, the dither mask D, shown in the upperstage of FIG. 17, is overlapped onto the frame F1. The record attribute“A” is then assigned to unit regions for which the numerical valueinside the dither mask D is no less than 8, and the record attribute “B”is assigned to unit regions for which the numerical value inside thedither mask D is no more than 7. The dither mask D is then overlappedonto the frame F2 and the same process is performed.

FIG. 18 is a plan view of the record attributes of the respectiveindividual unit regions that were determined for the respective unitregions inside the frames F1 and F2 by the method shown in FIG. 17. Ofthe 32 unit regions, the first record attribute “A” is provided to 16unit regions, the second record attribute “B” is provided to theremaining 16 unit regions, and the distribution of the record attributeshas a characteristic that is in accordance with the form of distributionof the numerical values in the dither mask D used. Thus, if unit regionshaving the same record attribute are to be positioned dispersedly or areoppositely to be positioned concentratingly at one location, etc., thedistribution of the respective record attributes can be controlledaccording to the dither mask D used.

Though an example of a case where the distribution factor fx=0.5 wasdescribed above, in a case, for example, where fx=0.25, attributedetermination is carried out so that the record attribute “A” isassigned with dither mask values of no less than 12 and the recordattribute “B” is assigned with dither mask values of no more than 11.That is, a dither mask D, constituted of a matrix in which 0 to (N−1)successive numerical values (or 1 to N successive numerical values)appear uniformly, is overlapped onto a plurality of unit regions of therecording surface Rec, the distribution factor fx, corresponding to eachindividual unit region, is used to calculate N×fx, and the first recordattribute “A” or the second record attribute “B” is selected based onthe magnitude relationship between “this calculation result” and “thenumerical value at the dither mask D position corresponding to the unitregion.”

A second method preferable for use in the attribute determining step isto determine the record attribute of each individual unit region usingan error diffusion method. Because this error diffusion method is alsowidely used in converting continuous tone images into binary images,only a brief description shall be provided here.

In general, whereas the distribution factor f itself can take on anyvalue between 0 and 1, there are only a finite number of unit regions towhich attributes are to be assigned. Thus, when, for example, anumerical value of 0.73 is provided as the value of the distributionfactor f for a portion of the recording surface Rec, and recordattributes are to be assigned based on this distribution factor to atotal of 100 unit regions, record attribute assignment at proportionsfaithful to the distribution factor can be performed by assigning therecord attribute “A” to 73 unit regions and assigning the recordattribute “B” to the remaining 27 unit regions.

However, if record attribute assignment based on the distribution factorf of the numerical value of 0.73 must be performed on a total of 10 unitregions, the fractional figure must inevitably be ignored. In this case,for example, a process of rounding the distribution factor f=0.73 anddropping the fractional figure of 0.03 so that f=0.7 and assigning therecord attribute “A” to 7 unit regions and assigning the recordattribute “B” to the remaining 3 unit regions is performed. However,accumulation of the round-off error of 0.03, which is dropped, may applyan influence that cannot be ignored as error. For example, when anumerical value of 0.74 is provided as the distribution factor f atanother portion that is adjacent to the abovementioned portion and thefractional figure of 0.04 is dropped by rounding so that f=0.7, theprocess of assigning the record attribute “A” to 7 unit regions andassigning the record attribute “B” to the remaining 3 unit regions isperformed and if the fractional figure is always dropped in this manner,errors due to the dropped fractional figure accumulate.

The error diffusion method prevents the accumulation of errors bydiffusing such errors. That is, in the case of the above example, if aprocess of dropping the fractional figure of 0.03 of the distributionfactor f=0.73 by rounding so that f=0.7 and assigning the recordattribute “A” to 7 unit regions and assigning the record attribute “B”to the remaining 3 unit regions is performed on the first portion, byperforming, on the second portion, adjacent the first portion, a processof adding the fractional figure 0.03 that was dropped in the firstportion to the original distribution factor f=0.74 to make thedistribution factor 0.77, then rounding this so that f=0.8, andassigning the record attribute “A” to 8 unit regions and the recordattribute “B” to the remaining 2 unit regions, the accumulation oferrors can be prevented.

Section 3. Record Pattern Preparing Method

The record pattern preparing step of step S40 in the flowchart of FIG. 3shall now be described in more detail. In regard to this “record patternpreparing step” of step S40, a description with reference to FIG. 8 wasprovided above in Section 1. That is, with the example shown in FIG. 8,the two original images Pic(A) and Pic(B) and the recording surface Recare positioned in the three-dimensional space, the interference fringepattern of the object light from the first original image Pic(A) and thereference light R is recorded in the unit region Ua, to which the firstrecord attribute “A” is assigned, and the interference fringe pattern ofthe object light from the second original image Pic(B) and the referencelight R is recorded in the unit region Ub, to which the second recordattribute “B” is assigned.

Such a process of determining interference fringe patterns on therecording surface Rec is performed by computing interference fringeintensities at respective individual positions of the recording surfaceRec. In performing this interference fringe intensity computation,various measures that have been proposed up until now can beincorporated. For example, the abovementioned Patent Document 1discloses a method for performing an interference fringe intensitycomputation while restricting spread angles of the object light, andsuch a method can be applied to the process of the “record patternpreparing step” of step S40 as well.

FIG. 19 is a perspective view of a method for restricting spread anglesof the object light in determining an interference fringe pattern on therecording surface Rec. Here, an example, in which a horizontal spreadangle of the object light O from a point light source G (a single pointconstituting an original image) is restricted to φ and a vertical spreadangle is restricted to ξ is shown. When such restriction of spreadangles is applied, the object light O from the point light source Garrives only within a restricted region S, indicated by hatching on therecording surface Rec. In other words, the information concerning thepoint light source G (the interference fringe pattern of the objectlight O and the reference light R) is recorded only inside therestricted region S on the recording surface Rec.

In comparison to such a method for forming interference fringe patternsoptically, with a method for computer holograms, because interferencefringe patterns can be determined by computation, hologram recordingmedia that provide various special effects can be prepared by applyingvarious measures in the computation process. Computation methods, towhich such various measures are applied, may be used as necessary in therecord pattern preparing step of step S40.

For example, as a recording medium that can display differentreproduction images to an observer according to the observationdirection, a stereogram, arranged by positioning a lenticular lens, afly-eye lens, or other lens array, has been known of old. JapanesePatent Publication No. 2004-264839A and Japanese Patent Publication No.2004-309709A disclose principles of preparing hologram recording media(CGH stereograms), with which the resolution of such stereograms can beimproved significantly. With these methods, by employing a method forchanging the radiance of the object light, directed from each point on avirtual object to the recording surface, according to the radiationangle, a medium providing an effect equivalent to the stereogram in thereproduction process can be prepared. Obviously, such a CGH stereogrammethod may also be used in the record pattern preparing step of step S40of the present invention.

That is, with the present invention, the “original images,” which areprepared in step S10 and are to be recorded in step S40, are notrestricted to simply geometrical virtual objects but broadly includessubjects to be recorded by computer hologram methods. Thus, “originalimage” data, as referred to in the present invention, not only refers toshape data of simply geometrical virtual objects but covers various dataused in the record pattern preparation computation of step S40. Forexample, if a method for restricting the spread angles ξ and φ, shown inFIG. 19, is to be employed in the recording process, the informationconcerning the restriction is also data that constitutes a portion ofthe “original image,” and if the above-described CGH stereogram methodis to be employed, the information on the radiance that changesaccording to radiation angle is also data that constitutes a portion ofthe “original image.”

FIGS. 8 and 19 illustrate examples of a method for recording eachoriginal image in the form of interference fringe patterns of the objectlight O and the reference light R (a method for recording each image asa normal hologram). In the case where this method is employed, indetermining an interference fringe pattern, based on an original image,for a unit region in the record pattern preparing step, the originalimage and the recording surface are positioned in the three-dimensionalspace, a predetermined reference light is defined, and the interferencefringe pattern formed inside the unit region by the object light fromthe original image and the reference light is determined by computation.

Meanwhile, with the present invention, an original image can also berecorded in the form of diffraction grating patterns. As mentionedabove, “hologram” in the present application is used as a broad conceptthat includes not only normal holograms, formed of optical interferencefringe patterns, but also includes pseudo holograms (diffraction gratingrecording media) formed of diffraction grating patterns. Althoughmethods for preparing pseudo holograms, constituted of diffractiongrating patterns, are known arts, such as described in theabovementioned Patent Documents 4 to 6, the principles of these methodsshall be described briefly below.

FIG. 20's show plan views for describing a method for recording a motifusing diffraction grating patterns on the recording surface Rec. FIG.20A is a plan view of an original image Pic to be recorded, which is atwo-dimensional image constituted of a pixel array of 8 rows and 9columns. This two-dimensional image is constituted from the three typesof pixels of pixels P1, indicated in white, pixels P2, indicated inblack, and pixels P3, indicated by hatching by dots. A simple motif isexpressed by the combination of these pixels.

To record the original image Pic, shown in FIG. 20A, in the form ofdiffraction grating patterns on the recording surface Rec, the samepixel array as that of the original image Pic is defined on therecording surface Rec as shown in FIG. 20B to make the pixels on therecording surface Rec correspond to the pixels on the original imagePic. Diffraction grating patterns, corresponding to the pixel values ofthe pixels P1, P2, and P3 on the original image Pic, are then recordedin the corresponding pixels P1, P2, and P3 on the recording surface Rec.

FIG. 21 is a plan view of a state in which the motif, corresponding tothe original image of FIG. 20A, has been recorded on the recordingsurface Rec, shown in FIG. 20B, by using diffraction grating patterns.The lines drawn inside each individual pixel shown in FIG. 21 indicategrating lines of a diffraction grating pattern for the sake ofdescription. The grating lines of an actual diffraction grating patternare recorded at a pitch of the level of the wavelength of visible lightand cannot be observed by the naked eye. There are three types ofdiffraction grating patterns in the individual pixels shown in FIG. 21,and these correspond to the three types of pixels shown in FIG. 20A.That is, at the position of each pixel P1, indicated in white in FIG.20A, a diffraction grating pattern, having grating lines that areinclined in the upper right to lower left direction, is formed, at theposition of each pixel P2, indicated in black in FIG. 20A, a diffractiongrating pattern, having grating lines directed in the verticaldirection, is formed, and at the position of each pixel P3, indicated byhatching by dots in FIG. 20A, a diffraction grating pattern, havinggrating lines that are inclined in the upper left to lower rightdirection, is formed.

By thus recording diffraction grating patterns according to the pixelvalues of the respective individual pixels of the original image Piconto the corresponding pixel positions on the recording surface Rec, themotif on the original image Pic can be expressed by diffraction gratingpatterns. Because the medium, onto which the diffraction gratingpatterns have been recorded as shown in FIG. 21, is not a normalhologram recording medium, a three-dimensional image cannot bereproduced. However, because diffracted light is directed toward anobservation position according to the diffraction grating patternsrecorded in the respective individual pixels, the three types of pixelsare observed in modes differing from each other, thereby enabling themotif on the original image Pic to be reproduced.

Although the medium shown in FIG. 21 should be called a pseudo hologram,in general, such media are also referred to as holograms, and asmentioned above, are referred to as hologram recording media in thepresent application.

FIG. 22 is an enlarged plan view of the diffraction grating patternformed in a pixel P1 shown in FIG. 21. A two-dimensional XY coordinatesystem is indicated in each of FIGS. 21 and 22, and the orientation ofthe grating lines is defined by a positioning angle θ of the gratinglines with respect to the X-axis. With the example shown in FIG. 22,grating lines L (black portions), with a line width d, are positionedinside a closed region v at a positioning angle θ and a pitch p. Thethree types of diffraction grating patterns shown in FIG. 21 correspondto changing the positioning angle θ of the grating lines in three ways.That is, with the recording medium shown in FIG. 21, the three types ofpixels P1, P2, and P3 on the original image, shown in FIG. 20A, areexpressed by diffraction grating patterns having three types ofpositioning angles θ.

Although diffraction grating pattern variations can thus be obtained bychanging the positioning angle θ of the grating lines L, diffractiongrating pattern variations can also be obtained by changing otherparameters. Specifically, different diffraction grating patterns thatgive rise to different diffraction phenomena can be obtained by changingthe line width d and the pitch p of the grating lines L shown in FIG.22. Also, although with the example shown in FIG. 22, the size of apixel is made the same as the size of the closed region v, in which thegrating lines are formed, by making the size of the closed region v, inwhich the grating lines are formed, 80%, 60%, 40%, and 20% the size of apixel, pixels, with which the intensities of the diffracted light are80%, 60%, 40%, and 20%, respectively, can be formed.

Numerous variations of pixels, each having a diffraction grating patternformed in the interior, can thus be formed by variously changing thepositioning angle θ, line width d, and pitch p of the grating lines L,the size of the closed region v in which the grating lines L are formed,etc. By using such variations to express the variations of the pixelvalues of pixels on an original image, the motif on the original imagecan be expressed in the form of diffraction grating patterns.

The correspondence between the unit regions on the recording surface Recand pixels does not necessarily have to be a one-to-one correspondence.That is, a single unit region defined on the recording surface Rec maybe defined as it is to be a single pixel and a specific diffractiongrating pattern may be formed therein, or a plurality of pixels may bedefined inside a single unit region and a unique diffraction gratingpattern may be formed inside each individual pixel. For example, ifsquare unit regions of 20 μm×20 μm are defined, each unit region of 20μm×20 μm may be used as it is as a single pixel, or an implementation,in which four pixels, each constituted of a square of 10 μm×10 μm, aredefined in a single unit region, is also possible. That is, indetermining diffraction grating patterns based on the original image forthe respective individual unit regions on the recording surface, one ora plurality of pixels is or are defined in each unit region,corresponding pixels on the original image are determined for thesepixels, and diffraction grating patterns inside the respectiveindividual pixels are determined based on the pixel values of thecorresponding pixels.

Also, scattering structure patterns may be formed instead of diffractiongrating patterns inside the respective individual pixels on therecording surface Rec. As mentioned above, by using diffraction gratingpatterns, a plurality of types of pixels that appear differently duringobservation can be prepared by changing the positioning angle θ, linewidth d, and pitch p of the grating lines L and the size of the closedregion v, etc., and the variation of the pixel values of the pixels onthe original image can be expressed by these plurality of types ofpixels. In other words, as long as a plurality of types of pixels thatmutually differ in appearance can be prepared for expressing thevariation of the pixel values of the pixels on an original image, theseplurality of types of pixels do not have to be formed by diffractiongrating patterns.

Scattering structure patterns are patterns with unique light scatteringcharacteristics and, in the present invention, may be used in place ofthe diffraction grating patterns described above. For example, JapanesePatent Publication No. 2002-328639A and Japanese Patent Publication No.2002-333854A disclose methods for forming recording media having uniquelight scattering characteristics by forming microscopic unevenstructures on surfaces. Surfaces of various light scatteringcharacteristics can be formed, for example, by roughening the surfacesof recording media by etching or use of chemicals or by performingmicroscopic embossing using an electron beam drawing equipment. Thus, bypreparing a plurality of types of scattering structure patterns thatmutually differ in light scattering characteristics and allocating aspecific scattering structure pattern in each pixel on a recordingsurface Rec according to the pixel value of a pixel on an originalimage, the information of the original image can be recorded in a mannersimilar to the above-described case of using diffraction gratingpatterns.

As described above, in the record pattern preparing step of step S40 inthe flowchart of FIG. 3, two types of record pattern preparing methodmay be employed. In the first method, interference fringe patterns ofthe object light from the original images and the reference light areprepared, and by employing this method, a normal hologram recordingmedium that enables a three-dimensional reproduction image to beobtained is prepared. In the second method, predetermined diffractiongrating patterns or scattering structure patterns that correspond topixels on the original images are prepared, and by employing thismethod, although a three-dimensional reproduction image cannot beobtained, a (pseudo) hologram recording medium that appears to shimmerbrightly or has a white, matted appearance is prepared.

Obviously, in recording the two original images onto the medium, one ofeither of the above-described two methods may be used or both methodsmay be used in combination. By selecting whether to record each of thetwo original images in the form of interference fringe patterns or inthe form of diffraction grating patterns (or scattering structurepatterns), media of the following three modes can be prepared.

In a medium of a first mode, both original images are recorded in theform of interference fringe patterns. To prepare such a medium, in therecord pattern preparing step, the first original image, the secondoriginal image, and the recording surface are positioned in athree-dimensional space, a predetermined reference light is defined (asmentioned above, reference light that differs according to each originalimage may be defined), interference fringe patterns of the object lightfrom the first original image and the reference light are determined bycomputation for the unit regions to which the first record attribute isassigned, and interference fringe patterns of the object light from thesecond original image and the reference light are determined bycomputation for the unit regions to which the second record attribute isassigned.

In a medium of a second mode, the first original image is recorded inthe form of interference fringe patterns and the second original imageis recorded in the form of diffraction grating patterns (or scatteringstructure patterns). To prepare such a medium, in the record patternpreparing step, the first original image and the recording surface arepositioned in a three-dimensional space, a predetermined reference lightis defined, and interference fringe patterns of the object light fromthe first original image and the reference light are determined bycomputation for the unit regions to which the first record attribute isassigned, and for each unit region to which the second record attributeis assigned, one or a plurality of pixels is or are defined in the unitregion, a corresponding pixel or pixels on the second original image isor are determined for the defined pixel or pixels, and a diffractiongrating pattern or a scattering structure pattern in each individualdefined pixel is determined based on the pixel value of thecorresponding pixel.

In a medium of a third mode, both original images are recorded in theform of diffraction grating patterns or scattering structure patterns.To prepare such a medium, in the record pattern preparing step, for eachunit region to which the first record attribute is assigned, one or aplurality of pixels is or are defined in the unit region, acorresponding pixel or pixels on the first original image is or aredetermined for the defined pixel or pixels, and a diffraction gratingpattern or a scattering structure pattern in each individual definedpixel is determined based on the pixel value of the corresponding pixel,and for each unit region to which the second record attribute isassigned, one or a plurality of pixels is or are defined in the unitregion, a corresponding pixel or pixels on the second original image isor are determined for the defined pixel or pixels, and a diffractiongrating pattern or a scattering structure pattern in each individualdefined pixel is determined based on the pixel value of thecorresponding pixel.

With the present invention, the original images to be recorded may betwo-dimensional images or three-dimensional images. Thus, in the“original image preparing step” of step S10, it suffices that digitaldata expressing a two-dimensional image or a three-dimensional image beprepared as an original image. Obviously, an original image, prepared asa two-dimensional image, may be recorded in the form of interferencefringe patterns or recorded in the form of diffraction grating patternsor scattering structure patterns. Likewise, an original image, preparedas a three-dimensional image, may be recorded in the form ofinterference fringe patterns or recorded in the form of diffractiongrating patterns or scattering structure patterns. In other words, itsuffices that, in the “record pattern preparing step” of step S40, aprocess, in which a record pattern of some form is prepared bydetermining (any of) interference fringe patterns, diffraction gratingpatterns, or scattering structure patterns based on an original image ofsome form, is performed.

An implementation, in which an empty image without an actual entity isprepared as one of the original images in the “original image preparingstep” of step S10 and no pattern whatsoever is formed for unit regionsthat have been assigned the record attribute of this empty image, isalso possible. For example, by using an empty image without an actualentity as an original image Pic(B) in place of the original image Pic(B)shown in FIG. 4B, a reproduction image, in which just the automobilemotif, corresponding to the original image Pic(A) shown in FIG. 4A,gradually fades out in space, can be obtained.

Section 4. Various Variations

Although the embodiments that have been described thus far are basicallyexamples, in which two original images are recorded onto the same mediumto provide an effect of blending of boundary portions of the motifs ofthe two original images as in the example shown in FIG. 2, the hologramrecording medium preparing method according to the present invention isnot necessarily restricted to a method for recording two originalimages.

For example, the basic philosophy of the art of the present inventioncan be applied to a case of recording a single original image. That is,in a case of recording a specific original image singularly, an originalimage preparing step of preparing, as data, the specific original imageto be recorded, a unit region defining step of defining and positioninga plurality of unit regions, each having an adequate area for recordinginterference fringes of visible light, on a hologram recording surface,an attribute assigning step of assigning a “specific record attribute,which indicates that the prepared specific original image is to berecorded,” to a portion of the defined plurality of unit regions, arecord pattern preparing step of determining, for each unit regionassigned with the specific record attribute, an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on the prepared specific original image to prepare dataindicating a predetermined record pattern to be formed on the recordingsurface, and a medium forming step of forming the prepared recordpattern on a physical medium are executed.

In particular, because with the present invention, a gradated motifexpression is enabled, in a case where a gradated motif expression is tobe carried out in recording a specific original image singularly, anoriginal image preparing step of preparing, as data, the specificoriginal image to be recorded, a unit region defining step of definingand positioning a plurality of unit regions, each having an adequatearea for recording interference fringes of visible light, on a hologramrecording surface, an attribute assigning step of defining a gradationpattern, expressing that an appearance probability of a predeterminedattribute gradually changes in space, and assigning a “specific recordattribute, which indicates that the prepared specific original image isto be recorded,” to a portion of the defined plurality of unit regionsthat is selected according to the appearance probability at eachindividual position when the gradation pattern is overlapped onto therecording surface, a record pattern preparing step of determining, foreach unit region assigned with the specific record attribute, aninterference fringe pattern, a diffraction grating pattern, or ascattering structure pattern based on the prepared specific originalimage to prepare data indicating a predetermined record pattern to beformed on the recording surface, and a medium forming step of formingthe prepared record pattern on a physical medium are executed.

Also, if a plurality M of original images are to be recorded on the samemedium, an original image preparing step of preparing, as data, theplurality M of original images to be recorded, a unit region definingstep of defining and positioning a plurality of unit regions, eachhaving an adequate area for recording interference fringes of visiblelight, on a hologram recording surface, an attribute assigning step ofdefining a gradation pattern for each of the M original images,respectively, which expresses that an appearance probability of a recordattribute corresponding to an original image gradually changes in space,and assigning any one of record attributes to each unit region accordingto appearance probabilities of the respective record attributes at eachindividual position when the respective gradation patterns areoverlapped onto the recording surface, a record pattern preparing stepof determining, for each individual unit region, an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on a specific original image corresponding to an assignedrecord attribute to prepare data indicating a predetermined recordpattern to be formed on the recording surface, and a medium forming stepof forming the prepared record pattern on a physical medium areexecuted.

Although in Section 1 to Section 3, specific embodiments for cases ofM=2 were described, the present invention can also be applied likewiseto cases of M=3 or more. For example, if the present invention iscarried out by setting M=12 and preparing a total of 12 original imagesand forming a layout, in which the respective original images arepositioned at positions of the respective numerals of a clock face, aneffect such that the 12 original images are blended with each other at acentral position of this clock face is obtained.

A hologram recording medium, prepared by applying the present inventionto a plurality M of original images, thus has a unique structure havinga recording surface, on which a plurality of unit regions, each havingan adequate area for recording interference fringes of visible light,are defined, and with this structure, image information concerning oneoriginal image, among the plurality M of original images, is recorded asan interference fringe pattern, a diffraction grating pattern, or ascattering structure pattern in each unit region, and appearanceprobabilities of unit regions, in which image information concerning therespective original images are recorded, change in space.

In particular, a hologram recording medium that is obtained with thesetting of M=2 has, as in the embodiment shown in FIG. 2, a uniquestructure having a recording surface, on which a plurality of unitregions, each having an adequate area for recording interference fringesof visible light, are defined, and with this structure, either imageinformation concerning a first original image or image informationconcerning a second original image is recorded as an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern in each unit region, and an appearance probability of unitregions, having a first record attribute and in which image informationconcerning the first original image is recorded, and an appearanceprobability of unit regions, having a second record attribute and inwhich image information concerning the second original image isrecorded, gradually change in space.

Section 5. Hologram Recording Medium Preparing Device

FIG. 23 is a block diagram of a basic arrangement of a hologramrecording medium preparing device according to the present invention. Asillustrated, this device includes an original image storage unit 100, arecord pattern preparing unit 200, a unit region defining unit 300, andan attribute assigning unit 400. The original image storage unit 100 isa component that stores, as data, a plurality M of original images to berecorded, and in the figure, a state, in which two original images of afirst original image Pic(A) and a second original image Pic(B) arestored as data, is illustrated. Meanwhile, the unit region defining unit300 is a component with a function of defining and positioning aplurality of unit regions, each having an adequate area for recordinginterference fringes of visible light, on a hologram recording surfaceRec. This unit region defining unit 300 can be realized, for example, bya component that, upon determining the length of one side of each unitregion based on an operator input, performs a process of automaticallygenerating an array, with which each single cell is a square that is inaccordance with the determined length.

Meanwhile, the attribute assigning unit 400 executes a process ofpreparing a “gradation pattern, which is defined for each of the Moriginal images stored in the original image storage unit 100 andexpresses that an appearance probability of a record attributecorresponding to the original image, gradually changes in space” (thismay be prepared based on operator inputs as well) and assigning a recordattribute to each unit region according to the appearance probabilitiesof the respective record attributes at each position when the gradationpattern is overlapped onto the recording surface Rec. The specificdetails of the record attribute assigning process are as have beendescribed above.

The record pattern preparing unit 200 is a component that determines,for each individual unit regions on the recording surface Rec, aninterference fringe pattern, a diffraction grating pattern, or ascattering structure pattern based on the specific original imagecorresponding to the assigned record attribute to prepare dataindicating a predetermined record pattern to be formed on the recordingsurface Rec.

The device shown in FIG. 23 can be realized in actuality byincorporating a predetermined program in a computer. This program is aprogram for executing the process of the “attribute assigning step” ofstep S30 and the “record pattern preparing step” of step S40 based ondigital data, expressing original images and prepared in the “originalimage preparing step” of step S10, and digital data, expressing unitregions defined in the “unit region defining step” of step S20.

1.-28. (canceled)
 29. A hologram recording medium preparing methodcomprising: an original image preparing step of preparing, as data, aplurality M of original images to be recorded; a unit region definingstep of defining and positioning a plurality of unit regions, eachhaving an adequate area for recording interference fringes of visiblelight, on a hologram recording surface; an attribute assigning step ofdefining a gradation pattern for each of the M original images,respectively, which expresses that an appearance probability of a recordattribute corresponding to an original image gradually changes in space,and assigning any one of record attributes to each unit region accordingto appearance probabilities of the respective record attributes at eachindividual position when the respective gradation patterns areoverlapped onto the recording surface; a record pattern preparing stepof determining, for each individual unit region, an interference fringepattern, a diffraction grating pattern, or a scattering structurepattern based on a specific original image corresponding to an assignedrecord attribute, respectively, to prepare data indicating apredetermined record pattern to be formed on the recording surface; anda medium forming step of forming the record pattern on a physicalmedium.
 30. A hologram recording medium preparing method comprising: anoriginal image preparing step of preparing, as data, a first originalimage and a second original image to be recorded; a unit region definingstep of defining and positioning a plurality of unit regions, eachhaving an adequate area for recording interference fringes of visiblelight, on a hologram recording surface; an attribute assigning step ofdefining a gradation pattern, which expresses that an appearanceprobability of a first record attribute and an appearance probability ofa second record attribute gradually change in space, and performing aprocess of assigning one of either the first record attribute or thesecond record attribute or not assigning either attribute on each unitregion according to appearance probabilities of the respective recordattributes at each individual position when the gradation pattern isoverlapped onto the recording surface; a record pattern preparing stepof determining an interference fringe pattern, a diffraction gratingpattern, or a scattering structure pattern based on the first originalimage for each unit region to which the first record attribute wasassigned, and determining an interference fringe pattern, a diffractiongrating pattern, or a scattering structure pattern based on the secondoriginal image for each unit region to which the second record attributewas assigned to prepare data indicating a predetermined record patternto be formed on the recording surface; and a medium forming step offorming the record pattern on a physical medium.
 31. The hologramrecording medium preparing method according to claim 30, wherein theattribute assigning step in turn comprises: a reference setting step ofdefining a distance reference line on a plane containing the recordingsurface; a distribution factor setting step of defining a distributionfactor f(x) as a function of a distance x that takes on values in arange of 0≦f(x)≦1; and an attribute determining step of determining arecord attribute to be assigned to each unit region in a manner suchthat, to unit regions, positioned on or near a positioning line, whichis parallel to and is at a position separated from the distancereference line by just a distance x, the first record attribute isassigned at a proportion of f(x) and the second record attribute isassigned at a proportion of “1−f(x)” or less.
 32. The hologram recordingmedium preparing method according to claim 30, wherein the attributeassigning step in turn comprises: a reference setting step of defining adistance reference point on a plane containing the recording surface; adistribution factor setting step of defining a distribution factor f(x)as a function of a distance x that takes on values in a range of0≦f(x)≦1; and an attribute determining step of determining a recordattribute to be assigned to each unit region in a manner such that, tounit regions, positioned on or near a positioning line, which is definedas a circumference of a circle of a radius x that is centered about thedistance reference point, the first record attribute is assigned at aproportion of f(x) and the second record attribute is assigned at aproportion of “1−f(x)” or less.
 33. The hologram recording mediumpreparing method according to claim 30, wherein the attribute assigningstep in turn comprises: a reference setting step of defining an anglereference point and an angle reference line, passing through the anglereference point, on a plane containing the recording surface; adistribution factor setting step of defining a distribution factor f(x)as a function of an angle x that takes on values in a range of 0≦f(x)≦1;and an attribute determining step of determining a record attribute tobe assigned to each unit region in a manner such that, to unit regions,positioned on or near a positioning line, which passes through the anglereference point and is inclined by just an angle x with respect to theangle reference line, the first record attribute is assigned at aproportion of f(x) and the second record attribute is assigned at aproportion of “1−f(x)” or less.
 34. The hologram recording mediumpreparing method according to claim 31, wherein in the distributionfactor setting step, a monotonously increasing function or amonotonously decreasing function is used as the distribution factorf(x).
 35. The hologram recording medium preparing method according toclaim 31, wherein in the attribute determining step, for a plurality Nof unit regions positioned on a same positioning line, a process ofusing random numbers to assign the first record attribute to N×f(x) unitregions and assign the second record attribute to the remaining unitregions is performed.
 36. The hologram recording medium preparing methodaccording to claim 31, wherein in the attribute determining step, aninteger ratio α:β that approximates “f(x)”:“1−f(x)” is determined foreach individual positioning line, and for (α+β) successive unit regionsamong a plurality of unit regions positioned on a single positioningline, the first record attribute is assigned to α unit regions and thesecond record attribute is assigned to β unit regions.
 37. The hologramrecording medium preparing method according to claim 30, wherein theattribute assigning step in turn comprises: a reference setting step ofdefining a two-dimensional XY coordinate system on a plane containingthe recording surface; a distribution factor setting step of defining adistribution factor f(x,y) as a function of two variables x and y of thetwo-dimensional XY coordinate system that takes on values in a range of0≦f(x, y)≦1; and an attribute determining step of determining positioncoordinates (x,y) for the respective unit regions and determining recordattributes to be assigned to the respective unit regions in a mannersuch that the first record attribute is assigned at a proportion of f(x,y) and the second record attribute is assigned at a proportion of“1−f(x, y)” or less.
 38. The hologram recording medium preparing methodaccording to claim 30, wherein the attribute assigning step in turncomprises: a distribution factor setting step of preparing a table thatdefines a distribution factor f, taking on values in a range of 0≦f≦1,for each individual unit region; and an attribute determining step ofdetermining record attributes to be assigned to the respective unitregions in a manner such that with each unit region, the first recordattribute is assigned at a proportion off defined by the table and thesecond record attribute is assigned at a proportion of “1−f” or less.39. The hologram recording medium preparing method according to claim29, wherein in the unit region defining step, a plurality of unitregions, having a same size and same rectangular shape and arrayed in aform of a two-dimensional matrix, are defined.
 40. The hologramrecording medium preparing method according to claim 39, wherein in theattribute determining step, a dithering process using a dither mask,comprising an array adapted to the matrix of unit regions, is performedto determine record attributes of the respective individual unitregions.
 41. The hologram recording medium preparing method according toclaim 39, wherein in the attribute determining step, a process using anerror diffusion method is performed to determine record attributes ofthe respective individual unit regions.
 42. The hologram recordingmedium preparing method according to claim 29, wherein in the originalimage preparing step, digital data, expressing a two-dimensional imageor a three-dimensional image, are prepared as an original image.
 43. Thehologram recording medium preparing method according to claim 29,wherein in the original image preparing step, an empty image without anactual entity is prepared as one of the original images and no patternwhatsoever is formed for unit regions that have been assigned a recordattribute of the empty image.
 44. The hologram recording mediumpreparing method according to claim 29, wherein in determining aninterference fringe pattern based on an original image for a unit regionin the record pattern preparing step, an original image and therecording surface are positioned in a three-dimensional space, apredetermined reference light is defined, and the interference fringepattern, formed in the unit region by an object light from the originalimage and the reference light, is determined by computation.
 45. Thehologram recording medium preparing method according to claim 29,wherein in determining a diffraction grating pattern or a scatteringstructure pattern based on an original image for a unit region in therecord pattern preparing step, one or a plurality of pixels are definedin the unit region, a corresponding pixel or pixels on the originalimage is or are determined for the defined pixel or pixels, and adiffraction grating pattern or a scattering structure pattern in eachindividual defined pixel is determined based on a pixel value of thecorresponding pixel.
 46. The hologram recording medium preparing methodaccording to claim 30, wherein in the record pattern preparing step, thefirst original image, the second original image, and the recordingsurface are positioned in a three-dimensional space, a predeterminedreference light is defined, an interference fringe pattern of objectlight from the first original image and the reference light isdetermined by computation for each unit region, to which the firstrecord attribute is assigned, and an interference fringe pattern ofobject light from the second original image and the reference light isdetermined by computation for each unit region, to which the secondrecord attribute is assigned.
 47. The hologram recording mediumpreparing method according to claim 30, wherein in the record patternpreparing step, the first original image and the recording surface arepositioned in a three-dimensional space, a predetermined reference lightis defined, and an interference fringe pattern of object light from thefirst original image and the reference light is determined bycomputation for each unit region to which the first record attribute isassigned, and for each unit region to which the second record attributeis assigned, one or a plurality of pixels is or are defined in a unitregion, a corresponding pixel or pixels on the second original image isor are determined for the defined pixel or pixels, and a diffractiongrating pattern or a scattering structure pattern in each individualdefined pixel is determined based on a pixel value of the correspondingpixel.
 48. The hologram recording medium preparing method according toclaim 30, wherein in the record pattern preparing step, for each unitregion to which the first record attribute is assigned, one or aplurality of pixels is or are defined in the unit region, acorresponding pixel or pixels on the first original image is or aredetermined for the defined pixel or pixels, and a diffraction gratingpattern or a scattering structure pattern in each individual definedpixel is determined based on a pixel value of the corresponding pixel,and for each unit region to which the second record attribute isassigned, one or a plurality of pixels is or are defined in the unitregion, a corresponding pixel or pixels on the second original image isor are determined for the defined pixel or pixels, and a diffractiongrating pattern or a scattering structure pattern in each individualdefined pixel is determined based on a pixel value of the correspondingpixel.
 49. The hologram recording medium preparing method according toclaim 29, wherein a size of each unit region is set to a size, by whichthe presence of each individual unit region cannot be recognized by anaked eye.
 50. The hologram recording medium preparing method accordingto claim 32, wherein in the distribution factor setting step, amonotonously increasing function or a monotonously decreasing functionis used as the distribution factor f(x).
 51. The hologram recordingmedium preparing method according to claim 32, wherein in the attributedetermining step, for a plurality N of unit regions positioned on a samepositioning line, a process of using random numbers to assign the firstrecord attribute to N×f(x) unit regions and assign the second recordattribute to the remaining unit regions is performed.
 52. The hologramrecording medium preparing method according to claim 32, wherein in theattribute determining step, an integer ratio α:β that approximates“f(x)”:“1−f(x)” is determined for each individual positioning line, andfor (α+β) successive unit regions among a plurality of unit regionspositioned on a single positioning line, the first record attribute isassigned to α unit regions and the second record attribute is assignedto β unit regions.
 53. The hologram recording medium preparing methodaccording to claim 33, wherein in the distribution factor setting step,a monotonously increasing function or a monotonously decreasing functionis used as the distribution factor f(x).
 54. The hologram recordingmedium preparing method according to claim 33, wherein in the attributedetermining step, for a plurality N of unit regions positioned on a samepositioning line, a process of using random numbers to assign the firstrecord attribute to N×f(x) unit regions and assign the second recordattribute to the remaining unit regions is performed.
 55. The hologramrecording medium preparing method according to claim 33, wherein in theattribute determining step, an integer ratio α:β that approximates“f(x):“1−f(x)” is determined for each individual positioning line, andfor (α+β) successive unit regions among a plurality of unit regionspositioned on a single positioning line, the first record attribute isassigned to α unit regions and the second record attribute is assignedto β unit regions.